Popular conversion engines

I've been mulling trying to use motorcycle engines for a while now but
have had dreadful luck at finding anything at all interesting on the
internet about the topic. I've found a few obtuse references to
snowmobile and water-craft engines but no solid leads. What should I
look at? I plan to implement a redrive, and I'm not sure what other
modification concerns I need to watch out for.

Price is the primary concern, followed by the ability to use mogas,
weight and fuel efficiency concerns.

I seem to remember there being one or two very popular forums online
for homebuilts, can someone jog my memory where I should be going?