If you can work out the length of the belt (normally printed on the belt), you may well be able to find a replacement from another car, even if the Tamiya belt is no longer available.

I'd back up the "shelf queen" route - biggest issue is going to be that the 415 was never designed around LiPOs and you probably won't get a LiPo in there without major modifications. Not worth doing. I haven't looked on eBay but you might find that a clean 415 is becoming collectable.