The BK 7,5 was capable of killing the Joseph Stalin tank. When shooting into the side of a T-34, it tore a 3ft dia hole. Firing only reduced air speed by 6mph. The unique mussel brake along with an adjusted powder load gave it good accuracy and didn't harm the modified airframe.
Hitech would have to introduce two versions of the Ms 129. A B2 with MK101 or MK103 and AP 20mm, or bombs only and AP 20mm. The MK103 had a higher rate of fire shooting the same ammo, 420rpm Vs. 260rpm, both guns from a 30 round drum. And a perked B3 with BK 7,5 with 12 rounds due to the one shot killing power of the BK 7,5 against tanks.
The B3 killing power would effectively make it a flying TigerII. I suspect an equal perk price would be fair. Can you imagine how the Hs 129 would get picked on by every red guy with a popgun for that?