Thanks everyone for the advice!
- The fan has been working well, it's nearly on all the time but it's been hot over here (northern california). No boiling over or smell coming from burning coolant.
- Plenty of coolant came out when drained thankfully.
- The gear oil looks good, not milky nor any unusual particulates.
- I took another look at the piston/head and it looks like something was banging around up there, so looks like I'll need a new cylinder head. I'm assuming you can't replace the insert in these stock ones? Perhaps it was a piece of the broken ring, I didn't see anything fall out when I disassembled.
- There's very slight vertical play on the crank, it was hard to measure but less than 1mm.
I'm leaning towards a "core exchange" swap with Jim at http://trialspartsusa.com, should be around $500 USD all said and done, including the piston kit.