The belt on the Savage is made by Bando, and it is available from industrial suppliers in various lengths.  You just need to find a supplier that will order what you want and sell it to you.

The Kawasaki EN500 also used a Bando belt, as did the KZ750.  They are a bit different in lengths - but I don't know if either one of them was long enough to work with your swing arm.

https://www.bandousa.com/

If you discover you're screwed the next possible answer is a chain??
It's not just a snap, and then you have to deal with a chain, but it's very well researched and explained.

Now I’m curious , I would like a longer swing arm so I could run 19” hoops. The trackers looks would be the only reason  , well maybe some better flat track tires too ! So who did you buy them from ?