Gunner Gasser can be made to fit as someone has fitted one to my 465. Works well. Haven't really looked into how it was done.
For the 250 I ended up paying way too much for the standard thing. Standard cable and everything. Kind of the cheats way out but .....
You can get everything NOS apart from the internal gear. The throttle tubes are hard to get hold of but but not impossible.
I still have the gunner Gasser copy throttle - just looking for a bike that the throttle fits on properly, then I will but that
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In the defence of the people on Ebay that I bought it off they did say it wouldn't work on a Yamaha. I thought "how hard can it be". Apparently too hard for me