I have had my STG-58 for around 8 years it's a stock weapon and I have had no problems with it so far!

I'm looking to upgrade parts of the rifle to make it more reliable or more user friendly still have the same stock on it! I did replay the cover with a rail. Thanks for the advice and help!

In for the discussion.  I mounted an ACOG on my SA-58 but otherwise it is stock.

If it's mostly Austrian parts, and built on a LMT-produced DSA receiver, then I doubt you'll be able to make it more reliable. 
What features are you looking to add? Modular hand guards add significant weight.

Aside from replacing the metal handguards with a plastic variety and removing the bipod (and keeping both safely secured), I wouldn't do anything. The FAL goes from lean and sleek rifle to heavy pig in no time flat when you start adding rails and forends. I shoot my surplus kit builds out to 600+ yds. with iron sights and don't feel the need to muck with it the specifics.

I would not change out handguards or anything like that unless you need to mount an IR laser. I used a GG&G 4 slot section to mount a light on mine for a while, and it sounds like you've got optic taken care of.
As others have said, the FAL gets far less appealing to carry and shoot when you start adding rails and such.

I was mostly thinking trigger or the DSA X Stock maybe upgrade internal parts make them lighter and stronger.

I'm not looking for rails or anything like that at all it would make the rifle a pig!

With a bit of work you can mount a Magpul MLOK rail section to the plastic handguards through the vent holes.  Lightweight and allows you to mount a light.  There is also a new MLOK handguard out now that weighs the same as plastic BC handguards or fiberglass NBC handguards.  No need to get heavy.  The only things I personally have use for mounting are a light and a handguard mounted sling swivel (I'm currently using a South African one that the SANDF uses on their DMR R1s that are designed to fit through the bottom vent holes).

If weight is a major concern for anyone, going with alloy parts is the way to go.  Going with FN parts is expensive and hard to source, but saves more weight than DSA, but both can help a lot in this area.  You can potentially get a rifle with light and optic mounted weighing about what a standard 50.00 weighs (using the weight of the Type 1s, like the G series era stuff, which isn't much more than 9 lbs.).

I have an early STG-58 that was pretty much perfect the way it was.  The only thing I could come up with was to add a folding cocking handle / knob.  Now it's perfect.

https://www.dsarms.com/p-14264-dsa-fal-sa58-usmade-metric-folding-cocking-handle-assembly-w-lever-undercut-lug-and-pin.aspx

Unfortunately, out-of-stock at the moment..

It's been reliable, you said it. Don't change any internal parts to potentially make it unreliable.

I have an original StG as well (over here in Italy is legal to convert MG into SA only)

When I first got it I replaced the recoil spring and other small parts from DSA and also replaced the handguard with a polymer one, removed the bipod, and installed a SAW grip.
The stock was replaced with a new one from DSA but still the same pattern  just because the original one was shiny and scratched.

Last years I also installed a nikon monarch 2-8x on a ARMS scope mount.

I’ve got one of the old DSAs from about 10 years ago with the matching Steyr parts.  Mounting a rail with modern optics just doesn’t go well IMO.    The only thing I did to MINE was to add an Ironwood walnut stock to it.  Looks greater than before.

I have a 21" STG58 of a similar vintage. I have also been contemplating mods to make it more  suitable to my needs.

So far, I have swapped out the fsp for a zero dot to get the zero I wanted. I also swapped the stock rear sight for another one with a larger aperture. This works well enough that I have decided hold off on getting DSA's A2 para rear sight. I may still get it latter to have both the large and small apertures instead of the medium aperture that my current setup has.

I plan to swap the metal handguards for the DSA plastic ones along with their matching rail section so I can put a light on it. I will probably ditch the bipod as I like how losing the weight of the bipod felt on my other FAL. I had previously installed an X-stock but I swapped back to the original as it seems to manage recoil better.

I also want to add a DSA extended selector as the current one is almost AK level user unfriendly.

I have the DSA railed top cover installed and I plan to add a T1 type red dot to it.

Only mods for my StG58 on a DSA type 1 was just the scope mount and a nice Leupold scope for it.

I have the rail top on mine also I'm mostly looking at upgrading the stock and replace the metal for arm on this just to make it a little lighter. Also like to find a good modern sling like a three point that would work for it. I don't thing my neck would enjoy a one point sling.

I'm not sure about taking the bi pod legs off it yet it will make it lighter but I have found them very useful several times wish they made lighter ones.

Scope I'm waiting and saving up for an ACOG for my baby! Learned to love them in the Marines just th  price of them is as much as a new rifle and it's kind of hard to spend the money!

Have other weapons I also want to add like a Glock 21, Lever action 45LC , 45 LC wheel gun and a long range rifle that I'm unsure of the caliber right now.

Honestly, you're not going to get much lighter than the steel handguards. They aren't that heavy. But they do get hot, as I understand it. DSA's bipod cut handguards might be a tad lighter. OEM plastic bipod cut handguards may cost you, but provide better insulation and are nicer than the DSA ones. The fibreglass ones are tougher than the plastic ones, but heavier than plastic or steel. The AIM Sports MLOK handguard is worth looking at.

For slings I use a BFG Vickers CAS. Works great. The buttstock adapter they sell would be of use, but it still works well enough without it.

I got bored with mine a few years back and made it a PARA, pricey but I like the final product.

I was looking at that but I like how accurate this rifle is! Back in 2008 I took this rifle to a friend a Marine Sniper instructor he took it out with his scope to 1,000 yards and put 10 in the chest and 10 in the head with a stock weapon! At that point woudk you care if the holes touched?

I don't I know this weapon is more accurate then me! Last thing I want is to make this weapon less accurate! Yes know catch 22! Yet I much rather have a rifle that is accurate to one that is more of a assault rifle. I have a few AR-15 to cover that.

Volley fire can do that for sure. Just takes a few hundred rounds.