You have other options. But that means replacing the guns or the gun sensors.
I use the TopGun LCD gun in my MAME cabinet, and that works fine. I.e. that I can use it for any shooter like Terminator 2, Operation Wolf, Under Fire or Cheyenne.
Maybe you can fit the new Ultimarc gun sensors in the original guns, which of course would be a better solution.
But please, if you convert it to MAME, do it as transparent as possible. Don't add joysticks and spinners etc and make it an ugly mutant.
If you keep the original gun housings, but with other sensors and a MAME PC, it would be hard to tell for anyone if it is the original or not.