So called "big bore" kits are common for 50cc scooters. You replace part of the engine block and sometimes the cylinder head. 50cc is pretty small when you thing about it so you don't need a massive change to get a performance boost.
Most of the time all you need to replace is the cylinder and piston. For two-strokes, this is often a very simple task. It might be as easy as removing four bolts and a circlip.
