While there probably is a nice disassembly out there to look at I will start with the cruder option.
Is there a hack out there which just adds the Japanese system to the NA title? If so then whatever it changes is the damage system.
While far from a flawless approach step one would be make a patch to revert the just Japanese system patch back to the NA base ROM (make it a simple IPS so no worries about verification). Chances are any hack which also added it in will use the same area (completely transplanting code somewhere else is possible but tedious and not many would do it for something as simple as this) so reverting that to stock should leave any levels/graphics/text/music/whatever alone but restore higher health.
If you have to dig deeper then I would still seek a simple damage system change patch and then attempt to understand what was done, and what the original system it just changed functioned like.
Even more crude is cheat based. If the emulator you are using has a conditional cheat option you could probably make something there which will detect a change from powered up mode to small and instead leave you just big.