Contact the folks at Motion Pro. I run pre-mix in my 250A and they made a nice one piece cable for me.

This is an Emgo from Niche Cycle , This one came with a cable that worked on a VM38 with the Yamaha adjuster on carb.
The throttle action is far superior to the stock Yamaha. The draw back is replace or remove the kill switch. The kill shown here is a tusk unit and like most is labeled backward for our application.
Most of the time I will get a long cable and trim to fit (modify). It takes some patience and a little talent. Basic tools will be some good cable cutters and a pencil torch. Lead solder (lower melting point than tin base) and flux. Flanders Cables sells just about every thing. I did get my splitter junction block apart on my 74 but be careful at this age the are very delicate.

A simple throttle cable is not too hard to make. I bought a generic kit that came with lots of brass ends, mix and match to your existing cable style. Worked fine while the oil pump was off the bike for repair.

Maybe this will work,

hvccycle.net/motion-pro-turbo-throttle-cables-70-1260/

Says for stock RD carbs, which i'm sure it is--for a 28mm carb. But i'm just having imaginary moments as an RD runs two carb so--umm--one throttle on each side of the handlebars. Never mind, back to sleep lol.

I gave them a shot at it and figured they would have record of one but said they dont. I was told between $115 and $135 for a cable.
I ordered a build my own from Revival cycles out of Austin that has lots of options included.