That’s a tough one. I’m surprised they don’t have more parts available on their website. Have you reached out to them to see what they recommend? I just looked at our Swagtron 5 and the throttle is definitely different, physically, from all of the other throttles here, so I don’t have a plug and play recommendation.
You’re right though, many throttles are likely to work, if wired in correctly, but unfortunately I can’t say for sure which ones. I’d check out Ebay and see if you can find anything similar. The good news is that random electric scooter throttles on Ebay go for about $12.