Just what is different between the two? Its basically the same with exception to the absence of combustable fluid injection into the cylinder and the addition of a special head..... correct?

Im wondering because Id like to try my hand at making a miniature air compressor pump.
It seems easy enough compared to an engine in theory.

Does anyone have any ideas where I might find some plans for one, so I can get an idea of what direction I need to be going in??

Or a downloadable CAD file on one???

Thanks in advance for ANY and all info, and sorry for being somewhat off topic.