Go with the .61, I put an OS .91 FX in my WM Dago Red. It would jump off the ground after about 5 feet of ground roll. Battery pack on the bottom of the fuselage, covered up with the scoop, no extra ballast. Went to a 3 bladed prop for ground clearance. Slowed down to a crawl on landing. Only headache is pulling off the head to remove the cowl.