I know next to nothing about aircraft engineering, so I'm just throwing things against the wall to see if anything sticks. In the end, I'm sure I'll end up sticking to something that's thoroughly established as a working combination, but in the meanwhile, I'm curious about solutions that could end up being low-cost. On that topic, a just-rebuilt VW aero conversion just sold for $700. Unfortunately, it was in Canada, and I couldn't talk him into boxing it up for UPS (about $300 delivered).
Could it work? Yes. But you would need some sort of reduction drive and a radiator and a suitable mount. Lots of work involved for sure. Also, parts for these old engines aren't widely available.
Probably far more trouble than it's worth. Not to mention, probably little or no established reputation for reliability as an aircraft engine, and for me, that's pretty much #1 priority.
I simply don't get it. People show the committment to spend lots of time and money building an aeroplane then skimp on the most vitally (yes, vitally) important bought in part. Don't. Buy a proper engine.