To straight out answer the question - a .12 Rossi pixie outlaw. it produces about 1.9hp at 42,000rpms. it will set u back about 250.00
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but the other question is why? there is no way the RS4 chassis can handle that kind of power - that vehicle was designed to be an entry level basher - not a pro racer. if u are serious about getting into racing - upgrade the chassi first. if u are just wanting to have fun - then stick to a more reliable upgraded engine - such as the OS .15 CV-R. that engine will make your car just scream, and will only set u back about 120.00 plus it is extremely reliable.