Just about any aftermarket opto-spinner will work. However, the Forgotten Worlds spinner had an extra axle/gear to increase the spin ratio. So while any opto-spinner will work, you may have to spin it more to get the full gun rotation than you would need to with the original spinner.
The original spinner was also a push-down spinner that activated the gun so it was a button and spinner in one. I haven't checked the pinout to see if the gun is just a JAMMA button 1 signal, if so you could use a spinner and button seperately. The problem with this is that you have to operate a joystick simultaneously so you'd have 3 things to control, might be an issue.
Strangely enough, I had an Arkanoid: R.O.D. conversion cocktail that I sold to a friend that had a spinner with the SEGA logo on it. It was taller than the regular Arkanoid spinner, so I'm assuming it was previously used for a pushdown button game. *shrug*
The original spinner, in case you're wondering, had the Capcom logo on it. The button-top joystick may not be a bad idea if you use a regular spinner, the game auto-fires anyway so it's not like the button needs to be repeatedly pressed.