An old brushcutter engine had carbon deposits in the top of the cylinder and piston, and in the exhaust port. It was decided to try to remove this because any dislodged pieces might damage the engine. For this I used some carburettor cleaner which slightly softened the carbon, a perspex scraper and fibreglass tip "pen". Of course it's essential not to damage the cylinder or piston, so using any tools harder than the cylinder/piston metal isn't recommended. The perspex is quite hard, part of an off-cut, and can be cut to shape with a hacksaw. The fibreglass "pen" is usually used to clean circuit boards etc., but worked well as a second stage tool here (the fibres are a health hazard to skin, eyes).
Of course this is also a good time to check the condition of the crankshaft, bearings, and wear on the cylinder, piston and rings, also that the ring(s) can move freely in their groove(s).
The engine seemed to run more smoothly after doing this, but it could be my imagination ... at least it ran.
The main tools used, plus carburettor cleaner