Try a 52cc twin from knoxhobbies..Engine Specifications

Piston displacement 50.8cc
Output 4.2 h/p
Bore 34mm
Stoke 28mm
Carburetor Walbo pump type
Spark plug resister
Weight 3.75 lbs with mufflers and ignition
Auto advance electronic ignition 4.8 to 6 volts
Propellers 20x8 8000rpm, 20x10 7100rpm,
22x8 7100rpm
Speed range 1400 to 9000rpm
Fuel 25 to 30 to 1 for break in period
Fuel induction reeds