Lowvoltage

I have a 1/3 Spacewalker that I haven't started yet, and I don't have an engine for it.  I see that a lot of builders use a G-38 or G-45 (and G-62) Zenoah for the kit, and that the Quadras and 50/55 cc engines have been used also.  I would like some opinions of what gas engine I could put on the plane, and expect reliable operation for years to come without underpowering or way overpowering the plane?

Thanks,
Joe