The goal is to create model engines with moving parts made out of easily found materials or 3D printer parts. Stirling engines can run off a candle flame. Steam, Rotary, and Quasiturbine engines can run off compressed air. Gas (Otto), Diesel and Jet engines are best turned with an electric motor. Single piston engines like a lawnmower can be taken apart and displayed. Odd motors like the ballbearing motor are dangerous and need more work on the safety precautions than the actual motor.
Engine conversion is illegal, but some are advocating flex-fuel conversions that would enable a competition among different types of fuel to power our cars.