I guess I'm wondering if there's a grassroots community of people doing engine swaps on their planes?
While possible, economically "it's not going to happen".
For non-type-certificated airplanes, there is a large grassroots community around "Experimental-Amateur Built" (E-AB) aircraft. EAA (www.eaa.org) is the largest owner club; they put on "Oshkosh" every year, which is the world's largest fly-in (of E-AB and factory aircraft). In E-AB, the constructor of the airplane has nearly full latitude to choose their engine setup (and to change it later). My limited experience suggests that those who start with tried-and-true aviation-origin engines have a higher success rate with their projects.
The Oshkosh eaa meet was always a lot of fun to go watch when I used to live in WI.
It's not the exact same engine, but it's the same technology - an air cooled 360 cubic inch horizontally opposed 4 cylinder engine. Magnetos provide spark, and as far as I'm aware the fuel injection is mechanical.
"Modern" piston engines used in aviation are essentially 1950's technology. They're surprisingly reliable for what they are, but innovation isn't really happening in the piston world.
There was an interesting upgrade being worked on by a couple of guys in Florida. It turns out that the engines in these planes have an expected lifespan of 2,000 hours or so, and can't use gas with ethanol in it, and replacement engines can cost in excess of $20,000; these guys worked on a way to drop in a $5,000 Chevy Corvette engine and greatly increase fuel economy, reduce noise, reduce vibration, and supposedly increase reliability. When they asked the FAA to sign off on it as a replacement they were told that without redundant spark plugs/spark sources it wouldn't be approved. This makes sense when magnetos are expected to be rebuilt every 500 hours, but it killed the Corvette engine replacement idea.
Regulation slows innovation. General Aviation proves it IMO.