I think the 9 was over kill. That's like going to a 46 tooth rear. Puts you right in the middle of the power. If you look at a dyno of montesa power, the graph goes almost straight up from idle.
I messed around, the 42 was almost good. A 43 was too low. So I ended up with a 9 and a 38. But I am on a 301 which is much quicker.