The HP to weight ratio has to be ideal. They used the aluminum Oldsmobile engine from the 60's because of it's light weight compared to cast iron engines of that era.
Now they are using the LS series engines from the Corvette. Over twice the HP of the Olds engine and similar weight.
There are 2 auto engines that I can think of that are STCd for aircraft installations. One is the Ford 260, the other an all aluminum V8 from the Olds F85. I would look for an all aluminum engine that is small to start with, and then try to find a reduction gearbox. Some use gears, while others use belts. These are necessary to slow the prop speed to 2700 RPM or less. Then you will have to come up with some kind of cooling system as they are liquid cooled.