What diggerdanh said.
a) Find a manual.
b) Adjust valves first. Measure compression, if ok then forget about it. If not, could be valves or piston.
c) Open cylinder, measure bore with a special tool you can probably borrow from somewhere (see youtube for example). If the ovalness is in allowed limits (see manual), you can get new rings only. If cylinder is oval, have the cylinder rebored to +1 size and get the according piston + rings.