ORIGINAL: RCDENT

I have posted a shot of several vintage engines. Top left is the Enya .60 II/III which was Phil Kraft's weapon of choice during the glory days of the Kwik Fli III. Top right is a Lee/Veco .61 from the same period. These were ''bypass-ported'' engines. On the bottom row is an O.S. .61SF and a vintage Rossi .60. The O.S. and Rossi shown are both ''Schnurle-ported'' engines, and because of the port arrangement with ports fore and aft of the cylinder, they are physically longer. The calipers are set at the length of the longer engines and shown next to the older engines. Also pictured is a backplate style mount used on the Kwik Fli III, which were about 1/8 to 1/4 in. thick aluminum.

You can see that if you use a Schnurle-ported engine and a later style mount, you will increase the length of the nose substantially, possibly affecting the appearance and balance of the plane. I would recommend sticking with a period-correct engine, or use a modern small-case engine such as the O.S. .55AX to maintain the original style of the airplane.