We are back in the world of guess the real question :-)How small?What is your aimWhere in the world are you.On the face of it your choices may be:ElectricityGasolinePropane (LPG)SteamDieselIf your talking modelsElectricityGlow fuelModel Diesel fuelOf these electricity is the easiest and best bet.
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. . . or looking even smaller ;- rubber band, electricity, compressed air, wound spring.
Ah - Push and down hill gravity racers.