Upgrade throttle tube?

I have a 78 650 c2 I just got back on the road and one of the first things I got to do something about is the throttle tube. I have drag bars with the correct length cable routed correctly but the throttle takes about 3 handfuls to twist. I have a supermoto bike and a common "upgrade" is to put a yamaha r6 throttle tube on it for a shorter twist. Is there a go to throttle tube for these old Kawasakis? Im using the stock orignial tube on the bike now.

I would guess that you will want to replace the entire throttle housing and switch assembly to get what you are after.

I've been thinking of putting in a shim made out of aluminum wire tie. That way the cable will have to climb it and the throttle will open quicker. Looks like there's enough room.
Steve