Quick update:
Looked at the head of the engine on Saturday, noticed the same damage on the head as on the pistons. This being the worst of it:
Possibly a result of swallowing a rock when IC piping blew off, or possibly a damaged spark plug, though the 4 in it now are all intact. My theory is that something damaged it a while ago, and the piston fractured only recently, which then lead to complete failure.
One of the flywheel bolt holes was quite messed up. One of the flywheel bolts was only 1-2 turns in the hole for transport and it got dinged, damaging the first couple threads. I didn't have the right tap, but a spare flywheel bolt and a cut-off wheel make a great tap to clean out damaged threads.
Popped the old engine/transmission apart with very little drama.
Pulled the oil pan off and found remnants of every gasket maker that has ever been used on this engine. Heh.
I got the spare engine all put together, cranked it over with the starter, and noticed that the oil pressure sender wasn't in place by a nice stream of oil coming out of the block. I tried to take the sender off of the old engine, only to find out that someone had threaded an 1/8in NPT sender into a metric hole. SAD FACE. This sort of killed any chance I had of getting stuff in and working over the weekend, so with a few more failed attempts to free the stuck valves on 3 and 4 I called it a day.
Prevailing theory on the stuck valves are clogged or stuck HLAs. I can compress all of the HLAs by hand, other than those on the stuck valves. Likely need to pull the cams out and clean all of them.