If it was designed back then, it might not have been designed to blaze around the sky, whipping through fast snap rolls, and cranking along at much more than a leisurely pace. Take a long hard look at the structure and keep that in mind when you decide on the engine for it.

Nowadays the .46 size engines pull like the old 60's did. etc etc....

got a picture of the thing? Identifying it might be the safest way to discover what engine would suit it best.