Thanks Tony, that's the first thing I thought of when engine possibilities were brought up. I'd want a fairly nose heavy airplane as a starter, but then my preference is usually a bit more nose heavy than most others anyway …
Using a 15 or 20FP, there will be no CG issue, because they are as heavy as a Fox or McCoy 35 was in the old, pre-muffler days.
But again, just use something that will start reliably in the range of a modern 15 on up, and it will be OK for its flight envelope.
Note that this airplane, like almost all of them from back in the day, suffers from the "old 35" = "modern 15" problem. When they said use a "35", they were thinking Forster, Ohlsson, Fox - not an OS, ST, Magnum, TT, etc. I am actually building an airplane in this size range - for a Hornet .09! Almost anything will work for this size airplane.
Brett