RE: CMP Katana 50
The engine practically fits within the cowl on a side mount...
Inverted the head sticks out the bottom.
The biggest problem is the muffler.
You would be safe going with a .61 through .75 for extra power. ( or even an AX .55 as Missleman suggested ).
All would work.
However you may want to base your engine decision upon the muffler arrangement and availability...
I'd be looking to putting in a pitts muffler for looks and to prevent fuel spray on the fuse...
So when things are somewhat close this may influence my choice.
IMHO this would be the easiest to mount.
Stock 2C mufflers are going to be a bit of a bear on the cowl cuts, because there IS room.... this forces the muffler to be at the same distance as the cowl.
A 4C engine and muffler ( as pictured ) are somewhat easy too.
It looks like they have an O.S. FS .60-.70 engine in the plane on the web site.
You would have to remove the muffler on this to remove the cowl.