True, I wouldn't go with a Franklin engine unless they started building parts and engines again. Maybe an O-200 or Lycoming O-235 with light wt starter and alternator. I've looked at the Corvair conversion too.Well, $1300 in 1970 would be $10,000 if this kit were sold today, if their cost increases mirrored general inflation over the last 52 years.
I think I'd probably choose something else if I needed 100-120 HP. Parts and good instruction (Mr Wynne) to build an airworthy Corvair derivative would be competitive with that. Heck, Bill Clapp (Azalea Aviation) will build and sell anyone a running 100 HP engine for $10.5k, or a 120HP engine for about $13k.