If you want to put the full crfx box in is expensive unless you find a whole one. My next time would simply be doing 4th and 5th. That is for two reasons. One is that if you look at the ratios, 2nd is the same for both rigs. Both make happy shifts from 2 to 3rd etc. if you put a light flywheel on a lower low it ends up being on the "darty" side. So I really don't have an ideal low gear anyway. Fifth with a 14 and 53 is good for mid 70's on any kind of dirt without it feeling too lazy, so I think I will be as happy with a 55 on it and the crf low gear. I started all this nearly 4 years ago and have to think sometimes of what I really ended up using.
I am always nervous about an extra load on a transmission. This engine hits harder than a stock displacement crfx by far. The primaries on the crfx are lower than the crf enough that I got concerned about breaking low gear, and went with the higher primaries. The lighter flywheel is easier on the transmission during hard shifts, but I didn't want to chance it not being enough on my first attempt.
Anyway, for me to do it fast easy and cheap I probably would try the top gears. You won't have to buy shafts.
Having some ph issues.