I have a saito 100 on my pulse 60 and <3 it, has great power and runs like a top. It balanced perfect and didn't need any extra weight. Saito 100 is actualy the recomended 4 stroke in the manual although im sure any .91 or the saito 125 would both work fine. I realy like the 4 stoke over the 2 stroke I had in my trainer and dont see my self ever having a 2 stroke again. If you go 2 stroke I agree with the others. 55 = 40 size, 75 = 60 size. The pulse is a great flying second plane and it will do great with a 2 or 4 stroke, its up to what you prefer.