In addition to the broken piston, looks like there's a crack running up the skirt. I picked up an 075 at a local auction for $20. Totally locked up, found the piston grenaded worse than yours, but the cylinder very intact. One $90 piston later and she is still running 4 years later on a chainsaw mill.
Like Andy said, the wore piston probably slapped hard enough to break the skirt. My 090 did the same thing, and was possibly helped along by using an impact wrench on the clutch nut.
All things considered, between the broken piston, missing gasket, hammered and wore cylinder, it's amazing you got it to run decent at all.
One caveat when you put it back together and use it: pay attention to the screws and maybe use blue loctite. These saws like to shake themselves apart. Mine likes to looses the recoil/gastank. Rebuilt my friend's, and his intake block gave me fits--took me 3 or 4 tries to get it to say together.
Chris B.