MVVS GRRT
The MVVS web site shows a carb for the rear intake GRRT, part#3226.
If MVVS is using the same carb mount dimensions as Nelson (I read somewhere this was the case), the OS carb #40A from the .46VX DF will fit. And it won't choke off your engine. It has an inner diameter of 15 mm!
RR