Do you have to drill and tap the cover? If so, I understand why you wouldn’t want to do that. Especially since the engine is all assembled and painted.
To be honest I never heard of vacuum VS pressure with fuel pumps either. But someone here posted a pic of the factory hoses and if it goes to the sidecover, there is no vacuum there. The crankcase makes pressure, not vacuum. You might need to research h50 fuel pump and see if that is indeed the factory location.
So running pressure to the pump will let you know whether you have the correct type of fuel pump or not. If using the sidecover makes no difference, then you may be able to rule out the fuel pump as the problem and then focus on the
carb instead. Diagnosis is usually testing things to rule certain things out of the equation. It could seem dumb but you have to start somewhere imho.
Where is the oil fill plug?
Can you use a bushing in the oil fill port, to install a barbed fitting in that location temporarily just to see if the fuel will flow correctly? If it flows correctly, then fuel pump is wrong and needs vacuum pulse. But if it it makes no difference, then it is time to look elsewhere.
This is how I would approach your problem if I were having this problem. Might not be the best way, but it will work.
I never used the round fuel pumps, so I don’t have firsthand experience with that particular fuel pump