The gearbox you linked is for Monkey engines, it will not fit the Grom...Please see tourist option on the attachment.. 0.8 final drive yes sprocket would have to be changed with it but not as extreme as 19:30 i wouldnt have thought
SP TAKEGAWA : Special Clutch & Mission Kit (Standard Clutch) [02-03-0022]
The Takegawa 5 speed for the Grom gives you the same top gear ratio as stock, Kitaco is even shorter.
SP TAKEGAWA : TAF 5-gear CROSS MISSION Kit [02-04-0293]
But I found a solution thanks to GearingCommander, just install a 17 inch rear wheel with a 17t front sprocket, it will get you there! :smile:
From the speed / revs, if speed was corrected, I think Kitaco was running a 17/34 or 16/32 combo on their Suzuka video with Neo bbk and 5 speed box.