In my intention to upgrade a CD50 from 50cc to 90cc, I made a video on what to look out for. The parts you need to upgrade:

– Crankshaft with a longer stroke (52mm);
– Oil pump and oil pump shaft;
– Gear ratio (optional);
– clutch (you need a clutch with more than one plate);
– and of course a bigger cylinder.

Watch my video about the project here:

A small sound sample of the 90cc with the famous Honda DH exhaust :