I wonder why the rocker box gaskets developed significant leakage in the first place.
The tightness of the bolts and nuts do need checking periodically. Both the headbolts and rocker box.
When the boxes have been removed to replace the gaskets, that would have been the best time to retorque the head (if its been running fine for years it likely doesn’t need it, but as you’re having issues I’d check it anyway).
Any new gaskets will compress a little over time, especially in the first 1000 miles after installation.
I’d suggest that you check all top end torques first, then re-set valve clearances, then see how it goes.
If the bike was off the road for some months, you have drained and replaced with new fuel?
You sound confident in carb synchronising, so I won’t waste time on that.
It would be unlikely to exhibit problems at higher throttle anyway, more suggestive of tight valve clearances, plug/coil possible but less likely.
Is it contact breaker ignition, or is this the first of the Lucas