I believe the cases are identical.The cr got a 5 speed box in -98,when the alu frame was introduced,I believe.It will be a big job,Im sure as you will probably need a complete gearbox.The kart people still use the 6-speed ,when they build a new cr engine,maybe a kart shop would be the best place to buy the parts.........Why would you do this?
A Pro told me that increasing a gear the CR will run more strong and the gap will be reduced. He also told me to wait that probably will be possible to bring a new gearbox from one CR prepared by an official team, but I am curious how they do that job...
I hope this makes sense.
You will need to buy... not only all the gears, but also the shift drum. This would be an expensive mod. The only problem with the 5 gear transmission is that Honda tried to space up the gears to keep almost the same top speed. Try a 53 t sprocket at the back instead. Alot of people on CRs at the track make the switch to a 53 and are happy with the transmission.
Most riders I know of like 1 tooth more on the rear sprocket,if they are not very light.My son is a very light rider and we usually run a 51 on a dry hard track......If you want to build a 6-speed cr,I guess it would be easier by far if you use an older engine in your frame......