I built a Great Planes PT-60 about 3 years ago. I used a Thunder Tiger 61 Pro two stroke.. The four stroke GP recommends for this kit is the FS-70 Surpass. For a trainer, I think the 70 would be plenty. I do not think a 91 would be "too much" though, so I'd probably go with the 91 if my future plans were for 60 size planes.