If this plane doesnt suffer from too many prop strikes Imay consider a big 4 stroke , then Iwould use a 12" prop. Ihate using a big prop on a tricycle plane since my landings usually suck pretty bad .

On my last 2 trainer type planes Iended up using 10" props until Iconverted the last one to a tail dragger (it died recently due to a radio failure).

Ibet my Enya 60 four stroke would be a great engine on this plane. Its a big heavy powerful 4S, if the ground clearance isnt an issue for a 12" prop.
Ido fly off grass a lot of the time and the local field is pretty rough. The next closest field is excellent and wouldnt be a problem except for the hour drive and resulting transportation costs.