Did you take the plugs out and try to turn it over, if it's been sitting it may be hydro locked with oil that has leaked in from the rotary valve seal to the cylinders.. Be sure you cover the plug holes with a towel if you try this.. the oil will go everywhere.. FI you ned to replace the motor I would stick with the 717, it's a pretty good engine.. And be sure to ground the plug wires so there is no spark..