There is a kit that is supposed to convert original arcade guns for use on PC's that HAPP sells. I don't think anyone has reported success with this kit. There was a lot of interest in the board, but since it was $200, I don't think anyone here bought it. I do think there was a thread in the last 3-6 months that someone discussed one of these boards they picked up off ebay. As I recall, it didn't work very good for their installation.
If you want to use the unaltered arcade guns today, I think that board is your only option.