A couple of thoughts on engine acquisition.

There are many sources for Rotax engines. After market rebuild/special builds don't always come with warranty.

Rotax engines from Kitfox have some specific things done especially for Kitfox installation. Two things specifically that I can think of is clocking the outlets from the water pump so they do not interfere with the engine mount. The other is the exhaust system, configured for the Kitfox Install. Oh, and with warranty.

Ralph