Carl,

Most important for the choice of engine is weight I think.
(I did fly the plane with a Merco 61 twin plug in the past.)

You can balance the plane on position of center of gravity and determine the needed weight of the engine.
Differences are a lot between the old engines and the modern ones, for example modern OS Max 61 FX is 534 gram.
Old engine, HB 10 cc 415 gram and OS Max H 60 417 gram.

When you have to ad weight in the tail to have the CG on right position the differences will be more than differences between the engines alone.
I did not weight the mufflers, only engines.

Second point is mounting holes if there are wooden bearers. Maybe you can find an engine that will fit in place.
Recommended power, 0.60, 10 cc as in the documents.
I do have a lot of engines, when you can give distances between the mounting holes I (we?) can look for engines that will fit in place.


Cees