There is one more thing to consider when doing an alternative engine - you really cannot treat the engine as a stand-alone thing.
Sticking to piston-engines for now:
1: Much of the mechanical reliability comes from running closer tolerances. These are only made possible by a rather constant operating temperature, which in turn requires liquid cooling.
Now if you don’t want to modify your airframe (or further down the line: don’t want your customers to have to modify their airframes), then all you can do is stick some radiator(s) up front in the nose. Where they don’t work well. You need them under/in your wings or in some P-51 style pod. There is a reason why a Kitty Hawk with its huge cooling opening would not cool well, and a P51 did.
So you need to modify your airframe.
2: You need computer power to run them, if you don’t want to be stuck with 1970s technology (and that’s already half a century ago). For that you need electricity. Your typical homebuilt does not have a system that is optimized for electric reliability. So you need modifications there. Engine busses, alternate electric sources, or any other strategy.
3: Unless you go with PWM pumps, an EFI system moves a LOT of fuel. Significantly more than mechanical fuel injection or a carburettor. If you don’t want to experience vapour lock misery, this means you need to place your fuel pumps very close to (or preferably inside) your fuel tanks. You really don’t want these pumps to suck the fuel in, as this pretty much guarantees vapour lock. Oh, and you need return lines. And maybe some header tank. So a redesign of your fuel system is called for.
4: You really need a constant speed prop for these to work well.
And by the time all is said and done, you’ve redesigned a significant part of your aircraft systems, plus a fair bit of the aircraft itself.
And then your next customer approaches you with yet another homebuilt design, and the whole process starts anew.
Like Ross said - this stuff is not simple and there is more to it than meets the eye.
When it works, it is soooooo good though!