So,

Got a KNS gas piston, and I'm trying to figure out how in gods name to remove the factory one. It looks like it was riveted in place and polished smooth. My attempts to punch it out have met with hilarious fail, and I'm not sure I can drill it out.

Anyone sell just bolt carriers without the piston attached? Everyone I've contacted (apex/centerfire) just has the fully assembled unit.
Any smiths that work on PSLs?

I can remove the old piston and install the KNS for you.

There is a transversal pin around a one centimeter from where piston actually starts. Often you can't see it because it's thoroughly polished or painted over. Normally it points exactly at the same direction as your charging handle. So you have to use a dremel tool to peel a paint or metal slightly at the place on the carrier where it is supposed to be. If you still don t see it , put some metal blueing on naked metal and you will see it immediately, because different metals are getting different colors then blued. Onse you found it  put the carrier in vise, take 3mm(sorry I am Russian and dont exactly know the imperial size) tungsten bit and drill the half way through . Then take a hammer and chase the rest of the pin , it will come out the opposite side easily. Then screw the KNS piston and pay attention to keep the same gap between the carrier and the piston, to let piston slightly move inside. it doesnt mean a shitty communist job, it is done on purpose.