Hi Chaps

I have an OS.50 2-stroke engine that's about 18 months old. I have looked after the engine really well, but lately it does not perform as it used to! It does not run badly but it does not peak. When I turn the prop over by hand it feels like there is no compression.

What could the problem be? Would it help if I replaced the gasket seal of the engine head? Or perhaps the backplate gasket?

Thanks