Which engine

[Eric Page]

...there are builders who just want to follow a known formula so they can get in the air. They don't want to work out the bugs of a cooling system or make a cowl from scratch. The build process can drag on for long enough so it's easy to see the logic there too.

Yep, exactly. I spent a ridiculous amount of time researching and diving down rabbit holes on alternative engines of every description before I finally took a step back and analyzed why I was building the plane in the first place. The answer was because I wanted a Kitfox to fly, not because I wanted to spend a couple of years chasing parts and fixing problems I don't know anything about. That realization, and the discovery that Kitfox Aircraft sells a very complete firewall forward installation kit for the Rotax 912iS (used on their S-LSA product) made the decision to use that engine a no-brainer.

I'd echo Alex's main point. If you're deeply interested in the experimentation aspect of this endeavor, and you're willing to endure some pain and trouble to achieve your vision of the perfect Kitfox -- or, if you're very knowledgeable about the type of engine you're planning to use -- then an "non-traditional" engine choice makes some sense. But, if you want a reliable plane to fly in a reasonable amount of time and with a minimum of hassle -- or, if you think you might sell the plane within a few years and want to have an easy time of it -- then a well-supported "mainstream" engine is probably a better choice.

Here's a cautionary tale. Alex and I have a mutual acquaintance who put one of the automotive conversion engines on his Kitfox. Because it's not an engine that had been used on a Kitfox before, he had to build the cowling from scratch. He had both coolant and oil leaks in flight, an engine stoppage due to a broken reluctor gear (on the ground, thankfully), and some kind of problem (ECU?) that made the engine refuse to start in the middle of a cross-country trip. He trailered it home, removed the engine and sold the plane.