Would a PPS 43 pistol build be CA legal?

I seem to recall that pistols with forward mags were banned.

It would have to be a bullet button before the new laws, which of course you can no longer do. Now it would need to be a break action top to load a fixed mag, not sure how that would work as I'm not familiar with the PPS 43.

I think it would be covered under the forward/separate mag prohibitions.  On a quick not well educated guess.

Only if detachable.

Or have been a bullet button rifle prior to 2017 that has to be registered by 2018.

It would have to be built with a fixed magazine that requires you to break the action to remove the magazine, though I would think the permanency of a welded on magazine that is then permanently part of the receiver would be the equivalent. It would be a beotch to load though but could be a single shot conversation piece. I have a few of these mangled by CA law firearms strictly for their friendly conversation value.

Also being a handgun if it is home grown and home rolled there are further regulations about unique markings and filling out a state form for ownership and recording those markings.

Probably the simplest would be to use a bolt of some kind to bolt the magazine to the receiver that could only be removed if you break the action open. I think you would have to weld some kind of tab to the magazine so it is not in the way of feeding.

If this was as popular as America's Rifle Model 15 or Model 4 then I am sure all the clever people would figure something much better so if you are one of those clever people then go for it.

Well, CA law bans handguns with:

Detachable forward magazines
Barrel shrouds

So I would have to make it fixed mag and remove the shroud.

An easier solution would be to make it a carbine with a kydex wrap, fix the stock, and 16" barrel.

This state sucks.

No, not possible in any way. Its a pistol that isn't on the CA Handgun Roster. 

You might be able to build one if you make the receiver yourself and use a parts kit but it'd still have to be a fixed mag, converted to closed-bolt, 10 round mag, and afaik you can't load that gun's mag while it's in the gun. 

So still no

Sorry, CA sucks like that