I'm not aware of any sensors in the cab. If you have any idea where they might be, Ill take a look. I'd have to assume that it uses light gun technology. I do know that HAPP guns can be used in HOTD (thats what's on mine currently), but from HOTD2 on up you have to use Sega guns from what I hear.
HOD 1 did not use gun sensors but I am not sure it can be just swapped into a Virtua Cop 2 machine. Ken at www.irepairsega.com can answer all your questions about all the Sega gun games. Give him a call, he's very helpful.
It is possible to convert the Sega gun sensor games (Jurassic Park and HOD 2) to use the gun system in Virtua Cop 2, which is a whole lot cheaper in terms of not having to buy the sensors and being able to use standard Happ guns instead of the ghastly expensive Sega guns.