Tech bit in plain english:
It's a 2- stroke. There are no valves, as the piston doubles as the valve. To ensure the incoming "blast" of air / fuel mix both fills the void & "blows" the exahust out the exhaust port, the piston has a "dam" across the top:
(random example from Google)
if the timing is advanced too much, the spark happens before the full "flush" has happened, resulting in more air / fuel one side of the dam than the other (the other side is still essentially non combustible exhaust).
Because the piston is it's own valve, if the spark happens too soon, at the "bang" moment the piston is still below the inlet / outlet ports, and so when the spark goes, the mix ignites onthe inlet side of the dam and creates a bigger "bang" on the inlet side than the outlet - net result the piston is pushed hard into the exhaust side of the cylinder, while still rising to compress the mix.
This results in a collision between the top corner of the piston and the spark plug edge of the exhaust port. The resulting debris gets wedged in the rings & makes a nasty mess.
Not that I know from experience or anything......
So, Replacement piston (Mallory / Sierra) - £75-100 or shedloads for an OEM one.
Replacement rings (for the other cyls) about £10- 15 for a set (maybe a little more if it's a V (more cyls than my Clamshell)
Rebore - I was quoted £120 for three cyls. Probably a little less if yours has a "proper" cyl head - mine is a blind bore design, which did complicate matters slightly.
The biggest PITA s dismantling it for the rebore. On the plus side, you don't need to do all the bores.
FWIW when mine destroyed itself (Locknut on the spark advance let go, so the adjuster screw wound itself out and the first I knew of it was when I opened the throttle and it stopped dead as the plugs were shorted by the metal that was chipped off the corner of the cyls!) I priced up a full rebuild. As Pikey says, it came to aboiut the same as a replacement engine (of unknown history). I decided to rebuild so I at least knew it was done properly. During the rebuild I fitted al lthe latest level parts, so dragging my engine form a 1970s build to a last production, which on paper at least should prevent it form suffering form all the Clamshell problems they were reknown for in the early days.