First thing to check is the carburetor. Is it the right one? A carb with a too-large opening will have fuel draw problems that will cause the engine to go too lean very easily. Adding more fuel pressure may help.

Another problem could be the fact that the engine's a bit worn. If the piston/liner fit is a bit loose, the engine will not have a good compression seal when it's at full throttle and fully warmed-up. This can lead to erratic running. When it cools down, the parts start to fit better again.

One other thing that could be a difficulty. Please check to see that the idle needle's not too lean. If it's too lean, it can interfere with high-speed fuel flow. Open the idle needle a LOT, start the engine at part throttle and see if you can then get the full-throttle mixture set. If it still misbehaves, then we'd strongly suspect the piston/cylinder fit.