1/10 scale cars use same size engines usually .12 to .18 engines fits..i think the kyosho cant use aftermarket engines without mods..the kyosho engines comes with a mount on the engine block..so u need to mod it in order to fit an aftermarket engine or find a mount that fits..the rs4 is a pretty decent car but im sure theres more out there but parts will be easy to find for the rs4..