First place I'd check would be RGVAC. The game is fairly common and odds are someone has the throttle out of a Hydro Thunder cab in their parts bin. Next you could watch Ebay for one or more likely the whole control panel to show up. Third, call some arcade distributors to see if you could get a replacement throttle unit. Fourth would be to improvise, PC aircraft sim throttle quadrants would be similar, or you could buy a cheap boat throttle setup, add some linkage and a pot to make one.