The AXE Rossi Cobra "VP" Modded is highest Off-road engine I've seen at 51,000 rpm @ 3.87 hp for a 3.5cc motor..
Here are the spec:
AXE Motor Rossi COBRA VP Modified Engine specs are:
DISPLACEMENT: 3.5cc
HORSE POWER: 3.87 HP
RPM: 51000
BORE: 16.00 mm
STROKE: 17.40 mm
PORTS: 7 + 2 BUSTER EXHAUST SYSTEM
CRANKSHAFT & TYPE: Æ 14 SG
GLOW PLUG: STANDARD
WEIGHT: 340 gr