Register it as an Experimental and put any damn engine on it that you want.
The FAA ( & the EAA ) closed the door on that kind of stuff a number of years ago now. Putting an Auto Engine in a Certified aircraft or hacking up an old Tri-Pacer to make it your "STOL Special" can't be done straight Experimental anymore. "Experimental Exhibition" is what you'd have to register it in. More restrictive, some say, do your due diligence, your mileage may very.