If it's been running fine for 2 years then suddenly not working then there's either something very simple wrong like a flat glow starter or on the last run something drastic happened like too lean and seized, mechanical failure etc. It's possible the head screws have come loose too. My rule of thumb for reasonable compression is to have a little fuel up in the cylinder then with a slow flick the prop should spin fairly quickly past the point where the exhaust port gets opened. An older rule of thumb was that it should hold some compression when held at TDC for about 5 seconds.

When you say it's an st-15 do you mean a Super Tigre G20-15 or maybe G15? If so, these have a cast iron piston so they need a fuel with at least 20% castor. Always use a gasket under the plug though.