My grandpa gave me a Chinese SKS that he bought back in the early 90s just because it was going to be banned. He shot it a couple times and it has basically sat in his safe since. When he gave it to me it had a 30 round metal magazine, he can't remember if he bought it like that or if he and my dad added it. Anyways, the metal mag did not work the one time I shot it (big surprise). A buddy of mine gave me a Tapco mag which worked for the ~15 or so rounds i put through it. However, I'd like to just put a factory fixed mag back in it and use stripper clips.

Was the gun modified to accept the removable mags or is it a simple process to reinstall the original mag?

What is a good source for an original mag? Are they all the same, or are there certain types to avoid?

I just want to keep this thing around because it's a nice change of pace compared to my other guns and I like being able to go plinking with cheap ammo. I know little to nothing about the SKS. Are there any parts I should change/upgrade that will help with reliability or should I just shoot away? I'm referring strictly to internal parts, not looking to swap out stocks, add rails or do any other gadgets.

Thanks in advance for any help you can provide this newb.

There is no modifications needed to add the duckbill aftermarket mag. You should be able to find a factory mag on ebay or gunbroker. All variants should work, but I'd look for a Chinese one if you could.  The SKS doesn't need any aftermarket upgrades to make them reliable, they are built like a tank and run best in factory form. Make sure you clean the bolt and firing pin very well before your fire it to much. There are videos on the internet that will show you how to do this.
Good luck returning your SKS to factory form, and enjoy shooting it.

Thanks so much for your help. I'm now on the hunt for an original mag and some stripper clips (and maybe a sling) and just shoot away. I also received 1000 rounds of ammo with the gun too. Not bad for free!

I've ordered stripper clips from here in the last 3 months shipping is costly but they work.

BXN clips

I did the same thing with my sks.  Factory mag and don't look back.

Wise choice.

Go on youtube and watch a video on the original magazine install. You will need to remove your trigger group first. Then insert the 10 round box mag. After that you pop your trigger group back in place.

Here is a great place to check out.

www.sksboards.com

This guy does it right.  An SKS I was given as a gift had no magaine- 10 rd or otherwise so I sourced one on ebay and used this video for installation instructions.  Very good with good angles on the gun as he's going through it.  A good original 10 round mag is getting hard to find from what I've seen.  I lost a number of auctions before getting lucky and getting a decent one for a decent price.

https://www.youtube.com/watch?v=pbiRnHlObqs

Try Liberty Tree Collectors for an original SKS magazine. I bought one from them a year ago or so, it was in very good condition with only a slight bit of rust on one edge. They still have them listed on their website
So you might be good to go.