One word of caution around Mazda's Skyactiv low comp diesel :
it is a turbo diesel !
edit : so volumetric efficency lost in low comp is restored because it is under boost
If building a 50cc engine, turbo is out because there is no appropriate unit available.
It doesn't mean you can't boost it but not with a turbo. (there you need to check the rules because the next best setup would be an electric blower on a battery)
If it has to stay N/A, then you're back to square one and a 18:1 CR.
In fact your benchmark setup is a VW SDi engine with corrections towards capacity applied.
http://en.wikipedia.org/wiki/SDI_(engine)
edit :
wich means you are in the 1.8 hp region.
also you need a fancy piston with a bowl in it to center combustion in the cylinder.