I believe the Franklin O235 was designed to bolt directly onto a Continental O200 mount.
As for the horsepower increase, the PQ-8A used the Lycoming O290 of 125 HP. I don't know if the fuselage was beefed up, but they sure put an ugly enlarged rudder on it.
If you are willing to go experimental, it might be economically feasible, but I think trying to get the FAA to bless it for a standard C of A would be very difficult (expensive).