maybe the reson you can't find the kit is because if you wanted reverse on your 99 mxz when you ordered it you got RER (electronic reverse) so there is no mechanical reverse to be found??
Our exclusive Rotax® Electronic Reverse (RER) can turn you around without the added parts or weight of a mechanical reverse gear. Now this touch–of–a–button technology is available on more sleds than ever before.
so there was no mech reverse available on the mxz 600 in 99