I've not long since had one of these cabs too, and was wondering the same.
From what I've read, the best options for using the Happ guns with a pc are to either put an Aimtrak module inside them or use a usb2gun board from GlobalVR.
The latter being quite pricey unless you find a used one.
Can anyone with the usb2gun / Happ guns / pc setup confirm how accurate it is as well please?