I think I may have isolated source of my low compression....
I guess it was probably running just a little too lean.
There's a complete motor on eBay that supposedly just needs rings that'll cost me $300 shipped. I'm tempted to go that way and part out whatever I have left after the swap. Might be cheaper and faster than dealing with this one.
Oddly enough, it still seems to turnover freely with no unusual noises, so I guess most of the melted aluminum probably blew out the exhaust. It doesn't sound like it's down in the bottom.