Short summary: cracked a head (maybe both), scored a cylinder and
piston; rods, crank, and case seem good so far.  We have an offer for
a $700 pig-in-a-poke air-cooled Vanagon engine.  What are our other
options for a new engine?

Long version:

Well, after we had our magic bus engine rebuilt in Omaha, we made it
about 200 miles before losing a lot of power on the freeway.  Didn't
know you weren't supposed to go over 45 for 300 miles after rebuilding
your engine.  I guess I would have known that if I'd rebuilt it
myself, because I would have read that part of the manual.  But we
weren't going over about 50 anyway.

Water flicked on the crankcase boiled immediately, even several
minutes after we stopped, and the oil smelled like hell.  It wouldn't
start for another half-hour or so (it had been having problems with
starting warm ever since the rebuild), and when it did, we drove very
slowly to the nearest town, Galesburg, about six miles away.

Compression test said 45 psi on cylinder #3; the other three were fine
at around 100.  We rented a U-Haul and towed it up to Minneapolis to
take the engine apart.

Looks like that piston and that cylinder are pretty badly scored, and
one of the rings is stuck in its groove, and there's a crack between
the valves in the head.  My wife uploaded photos of the whole removal
and disassembly process:

http://flickr.com/photos/blmurch/sets/72157594187260002/

The other piston on that side has a little bit of scoring.

I haven't taken apart the other side of the engine yet.  I don't know
what I'll find; maybe a good head.

Looks like the thermostat is missing, along with the cooling control
flap on the right side; the flap on the left side was covering the oil
cooler all the time.  I have the impression that the oil cooler is
supposed to have air flow through it when the engine is hot in order
to cool the engine more effectively, but clearly that wasn't
happening.  Perhaps this explains why it only ran for 200 miles.

So since I need to replace at least a head, a piston or two, and a
cylinder, I'll probably be better off with a new engine altogether.
We have a lead on a running air-cooled Vanagon engine with unknown
history for $700; how much time would it take me to install some other
kind of engine in here, such as a diesel, water-cooled, Subaru, or a
Jetta or Golf engine?  Would it cost a lot more?  Are there other
air-cooled engines out there with more-accurately-known history for a
similar price?