The Enya .60 II or III runs like a jewel, and new ones occasionally turn up at online auction sites. K&B / Veco .61 is another good choice and "in period". If you find an original Veco, you can still send it off to Clarence Lee and get it customized. The original Kwik Fli III was designed to use a backplate motor mount with the above-mentioned engines. When the engine manufacturers moved to Schnurle-ported designs, the engines all got physically about 1/2 in longer (necessary for the port configuration). While these engines make incredible power, trying to shoe horn them into a KF will require lengthening the nose and changing the look of the plane.

I would try to stick with a vintage .60, or a modern .46-55 engine to keep the engine size and weight in proper perspective.

I just wish somebody would locate a stash of Du-Bro 14 in canopies (used on the original KF III).

Good luck, it's a great bird!