Welcome back Marc!

Not long abgo, I had a similar problem.
I believe your engine is not getting enough fuel at high rpms, in other words, the engine sees a lean mixture at high rpms.

As w8ye stated, there are many little things that may contribute to that, from the tank to the spray bar in the carburator.

Check for two things:

Air leakeage into the crankcase and carburator: This could be from harden gaskets, needles, barrel, flat bearings, etc. I suggest to check / replace all you can.

Fuel blockage: From gum and hardened oil inside the spray bar, nipples, etc. I suggest to soak parts in fuel for some time, and blow with compressed air. Do not use wire or any metal tool.

The o-ring you mention is highly suspicious. You could help it with teflon tape applied to the thread.
Diassemble the spray bar and blow in the opposite direction in order to detect air leakeage thru the o-ring / thread.

Use a new glow plug and new fuel.

Regards!