Ok, measured stuff and ordered some parts. And called the original saw owner while I was at it.
This saw wasn't the big bore saw after all. He couldn't remember for sure either, so I did some measuring. It is the original cylinder that was ported and polished. Original piston, he says he re-ringed it a few times probably but can't remember for sure.
I figured it wouldn't hurt freshening the motor up, so I ordered a piston/ring kit, a carb rebuild kit, spark plug, and all needed gaskets. The porting looks great to me, I see no reason to not run this cylinder. All he did was smooth out the intake and exhaust tracts, no material removed from the cylinder wall itself.
I also ordered a better compression tester, and a Tachometer while I was at it. The Tach is on back-order.
Once everything gets here I'll put it together, along with all new fuel lines. From there I'll see where I'm at, but I see no reason why it shouldn't be fine. I doubt there's an air leak, because the piston seems fine but worn. Wouldn't an air leak cause a lean condition, leading to more scoring?