I think the regal reg would fit size wise but the single piece tube is going to be an issue for removal later as the valve has to be removed with a special tool from the filler side, with a reg in the way that would be..... tricky unless you in some way make a device to pull the reg and plenum chamber back out again
also you would need to drill a ventilation hole in cylinder at exactly the right spot to allow the reg to breath, all possible but not easy to get right and very easy to get wrong,