RB's are made to rev, in the words of its designers its better to over rev and aid cooling than be over geared and over heat, that gearing is ok if ur 12 stone, but I tried 4.7 & 4.8 and it will not pull up hill in to a light breeze, try it and see. I did I went back to 5.22 and it flies hits 80 no problem and im a little bit heavier than the OP. Just give AFs a ring and see what Ben says, they recommend Indian GP200 gearing and std sprockets or 47 19.
Anyway dont take my word for it just try it and see for yourself best way to learn.
I know it was the 250 but replay scooters had a RB250 done by Jb that had 17/46 and a SX200 box (4.20) and that pulled it easily, but it depends on the rest of your set up
TS1 and PX stay in the garage (future barn finds) out on the GTS 300 when I go out on a scooter.
I know I’ll get there and back