Well it sounds like lean running condition, but the fact that it came on after the head swap makes me think major compression leak at the head gasket.
Do you have a compression tester?
Does the up jetting make any difference at all?
In my experience when a bike can't make any torque its usually running lean.
Does it rev super hi just make no power? Or does it just bog out?
If it really just bllaaahhhs out, thats pretty much guaranteed lean running, as in too much air or not enough gas.
Is your carb clean?