Depending on what hull you're looking and I'm kind of guessing here that it may be a Prather, the deck is too high for an air-cooled engine to get enough air to keep it cool.
The other problem is that with most .21 engines made for cars and trucks is that most have slide carbs and the wrong shafts for marine use. Slide carbs just make hooking up the linkage harder but the big thing is the shaft off the crank. The SG shafted engines don't adapt well. you have to cut off the unthreaded part then you really don't have enough threads sticking out once you install a flywheel to hook up the flex cable collet to.