These are quite a rare commodity. In all my time with twins I have never seen a new one. No one makes an aftermarket version and as far as I am aware these are unique to our twins.
Like all things with moving parts they do have a limited life span and as the starter is unprotected at the front of the engine they get probably more than their fair share of abuse.
If you manage to find a 2nd hand one (they do come up) don’t forget you can never know what it’s lifespan is likely to be,
The best way forward is to refurbish the one you have. The parts to do this are available but they are not cheap and a bit of skill and ability is required to do it correctly so if you don’t have the confidence get someone to do it for you. I will keep an eye out for one for you.