I looked into the idea of putting the internals of an Act lightgun into a blue Namco gun body a few years back.
What I worked out was the electronic from the Act gun should fit in the Namco Shell without to much issue, but you would need to take care with the placement of the sensor, and you would probably want to extend the cable to the gun.
Now that's the easy part you've got a light gun that looks like an arcade gun but no recoil, the best option for recoil I believe is to retain the Namco 24v recoil Solenoid assembly, problem is how to make it work: (you would need an additional 24v power source to retain the Namco Recoil assembly)
The best option would be if be if M.A.M.E. emulated & mapped the light recoil signal to a serial port (it doesn't to my knowledge, please correct me if I'm wrong), then you could with a simple relay or transistor circuit attached to the computer send the 24v pulse to the solenoid in the gun much like the original arcade machine.
The only other piratical alterative is to make a small custom circuit probably based around a 555 timer and a transistor type relay setup (don't ask me for a circuit diagram I never got that far) basically you would connect the circuit to the trigger, and the circuit would in turn send the trigger sign to the Act gun board, and at the same time send a timed 24v pulse to the recoil solenoid, pull the trigger the lightgun fires and the recoil kicks at the same time, you could in theory extend this so that if the trigger is held down for more than a set time, the circuit pulses the output thus you have full auto.
I haven't had do much with M.A.M.E. in a few years, but if I was planning this now I would probably use the internals of an EMS LCD Topgun, simpler wiring setup, not tied to CRT monitor so more flexible, already has vibrate which could be used as an output signal to a custom 24v Solenoid control board.
Anyway good luck, I'll be very interested to see how you get on.