Remember, 2 stroke spark plugs usually stop working properly after not very many hours, due to the effects of oil deposits on the insulators. It's best to replace them, but if you can't (for example, 18 mm spark plugs are a bit hard to get) I've been known to scrape the insulators carefully right down to the root, then clean with carburetor cleaner - an awesomely powerful solvent.
In the old days most garages had Champion spark plug testers, which apply air pressure of anything up to the shop air supply pressure of 80-100 psi. However the machines also had a sand-blaster built-in, to clean the spark plug insulator. Do not let them sand blast your plug - it removes the glaze from the insulator, and makes it foul very quickly. Inspect your plug, when you've cleaned it properly with solvent. If the insulator is rough and unglazed, get rid of the plug.
I suggest you go hunting for a piston ring or two, of the correct width and depth, and which gives an appropriate ring gap in your cylinder bore. If necessary adjustments can be made to the peg recess in the rings by careful use of a needle file. However if the piston is scored significantly, this isn't going to work, a better piston will be required.
