I built and flown about 5 different Seamasters over the years, Arf's and kits - with engines from ST 40, 45, OS 46 and currently Enya 50 CX. They all worked well when propped right. The plane is very well designed and can handle just about anything, engine and flight wise. I also have a Northstar and J-3, but like the Seamaster the best. If I were to go 4 stroke, anything between 52 and 65 would be fine - here's a pic of the Northstar -