I stumbled across these Ultimate 8-way joystick conversion kits from the early 90s in the warehouse this weekend, so I've added them to the Flea Market page. There are 5 red sets & 5 blue sets available, but you could use your existing Happ Ulimate stick's handle with the kit if a different color is desired.

The kit consists of a handle, z-stop, 8-way actuator & 2 pivot cylinders. It differs from the std setup in that the z-stop has a sleeve molded into it on which the actuator is mounted providing a smoother, speedier rotation. You do have to remove the bottom bracket in order to change the z-stop.

The handle & pivot cylinders are the same as used on the traditional Ultimate stick & I suppose you could cut the sleeve off the z-stop to use as a replacement on the std stick if parts are all you're looking for.