Better yet get a second hand 2015 CB300F Honda and throw in those Grom wheels and you will have 29-30HP mini-bike with the ability to ride on the Freeway since most states allow 150cc and over to be on the Freeway. You will have 6 speed, water cooled, more gas to carry and a fun project. A CB300F is around $3,999 plus tax and DMV add on's and would be cheaper than modding a Grom and buying a BBK engine which cost around $2,400-2,600 plus your Grom $2,999 so you are looking at $5,600 plus and this dos not including fuel manage system and all the mapping, exhaust system and air box system that you need that will make your grom faster.Ive got an idea for my next sport bike build to put Honda from wheels on an 08 cbr125r coz that would be mad af. Would this work at all?
Going from 12" Grom wheels to 17" CBR125R wheels would actually RAISE the bike 2.5" (6.35cm) at the axle. That would give you a 32.6" (828mm) seat height! Center of gravity would be raised considerably and could make handling a bit squirrely....You'll also be lowering the bike a lot. Expect a corresponding loss of ground/cornering clearance if that doesn't stop it working completely.
Front caliper spacer could be as easy as a piece of metal (fairly thick steel I'd say) with four holes in. You only need to move it down in a straight line by 28mm after all, it's not going to be a complicated cnc project.