I agree with Pe that a higher pitched prop, if turned at the same revs as a lower pitch of the same diameter, will give more thrust but of course the engine will need to be developing more HP to do that. The usual statement that thrust sepends on diameter and speed depends on pitch is really a gross over-simplification. Hence the myth .

Engine breathing (as the high school shop teacher said) is the first step in getting more power and that kind of equates to a "port and polish" starting at the carb and working through the rest of the engine. In the OP's case, wanting more HP at the same revs, the goal here is to increase the torque at the same revs (which also means getting more HP at those revs).

The second step, after the "port and polish", is to make sure the combustion chamber area makes the best use of any increase in breathing. That involves things like having a close squish area, correct size squish band and combustion chamber shape (all of which add to give the highest compression the fuel can handle).

I'm not sure what was intended to be meant by a 2 stroke needs back pressure to operate correctly but the less back pressure in the exhaust the better. An open exhaust is best but way too loud (noting the OP didn't want to go to a full tuned pipe). If it referred to increasing the exhaust timing then that'd be the wrong way to go as far as increasing torque at the same revs.

And it's likely that I'm over-simplifying things here a bit too .