I looked at the Vario Skyfox mechanics you are using. The look very high quality and well designed. I think the whole problem is the fact you are 6000 RPM shy of the 2 stroke RPM. The extra RPM of the two stroke will move a considerable amount more air.
I was concerned at first the engine would be breathing hot air but the design puts the carb on the cold side so there should be no issue there.
If this were mine, I would make sure the fan is very tight in the shroud to reduce tip recirculation to an absolute minimum. Seal all the gaps that will allow air to leak in after the engine and block any opening that does not directly let air to the cylinder of the engine. Maybe direct more air to the cylinder head if possible?? If the crankcase is open to air can you block it off to maximize the air to the cylinder?
I'm sure you have done all this already but I thought I'd throw it out there anyway.
Having the fan suck hot air is less efficient that blowing cold air. But you already know this. Too bad you can't rig some sort of water mist upstream of the engine to make use of some evaporative cooling???? Just what you need, more complication