o, i'm sure that gear is in a lot more models of bikes than just the 954 or 929.

its about wear pattern. it doesn't matter that they're the same when they are made. its after, lets say 15,000 miles, the way the 2 gears wear on eachother is unique. the mating gear isn't worn out, but you know, to some extent after the first time the engine turns, there is some "wear", very minimal, but its there, and there is a pattern that the two gears make.

if you put a brand new gear into the transmission, it will cause both the mating gear (that already has one wear pattern) and the new gear (that has no wear pattern because its new) to wear out 2x as fast as they make a "new" wear pattern, and it will shift very hard because they haven't broken into eachother yet. think about when that gear slams into place with the mating gear. there are some extreme pressures at play there. in an article in sportrider magazine, they mentioned something how on some racebikes, gears in the transmission were regularly seeing 100,000 psi between the teeth under full load.

now look, if you wanted to throw just the new gear into the tranny, it can be done. it will work. its just that it'll be hard shifting while the new gear seats into the mating gear, and it will cause those two gears to wear much faster. so for the best results, you should get a good used tranny off of ebay and throw the whole thing in there.