The DC brushed motor you listed is a big multi pole that has a low kV/RPM and your engine is likely to put out it's best torque and power above 5000 rpm. The 49cc engine will weigh much more than the 25 cc engine, but should be a closer match to the DC motor. In either case you will need a diode in the wires out to prevent the battery from trying to spin the motor when the engine is off. The brushless AC motor has a more efficient power output for power in, but will need 3 bridge diodes divided between all 3 legs of the wires, since it outputs only AC current.