My old trike had the welded on sprocket to the hub. That didn't last long. I end up ordering a new rear hub with 2 sprocket on it and adding mm adapter to it. That was for the 2 stroke set up. Should work just fine for the 4 stroke set up. Make sure you get the hub with the 2 sprocket closer to the pedal side as you really dont have much room on the hub. I am building up chopper trike with 4 stroke kit. Good luck!
I hope I'm not too late to help with some advice. I just signed up to this site yesterday. Anyway, Check out Staton Inc. I think they will have what you need.
Thanks,
Fatdaddyred.