Nice images. Looks as though it’s just been restored, has it run “normally” before or is this its first outing with you? Are you sure that this is not the normal behaviour of an ungoverned two stroke? I’ve got a couple of machines with similar engines and if the throttles are opened more than about 1/3rd I have to run after them!
I think that the easy starting rules out air leaks on the induction side or crank case so perhaps you could try richening the mixture a bit – raising the needle in the throttle slide.
After that I’m at a loss as what to suggest.

As WP say’s about an ungoverned engine yes this is normal, put the machine into work cutting grass and the rpm would drop pretty quick then you will find that you would need to open the throttle to get it to cut, the heavier the work the more the throttle need’s to be opened, if the engine was a four stroke doing the same work it would be governed and the throttle would open it’s self dependent on initial throttle setting when under load, when not under load the rpm would be limited by the govenor, which you engine has not got hence the rpm get’s fairly high, the torque figure is higher at a lower rpm on the four stroke and it is normally quieter as well, because it can be run at a lower rpm figure.