That is Detonation from using either Old Gas, or Low Octane Gas for your Compression Ratio used. What CR Ratio are you running? For most Stock 2 Strokes you need 91+ Octane, if it's CR has been altered higher you probably need an even Higher Octane. Did you do a Cranking Compression Test, if so what psi was it? Did you do a Squish Band test to see what it was, if so what?

Pump Gas can loose 2 Points in Octane in as little as 2 Weeks. 100LL Aviation Fuel has a 5 year shelf life.

There is different ways to fix your head.
1. Weld up Chamber and re-machine.
2. Machine Bottom and re-cut Chamber till cleaned up.
3. Machine Head for Inserts like most Billet Heads use.
4. Sell your Stock Head and Buy a Billet Head.

I have bought NOS Billet Seadoo Heads off eBay for $29 to $75.

Just my 2 cents!