I couldn't find a rebuild kit.
Check out post #141 here:
http://www.3wheelerworld.com/showthr...ghlight=tecate
A 93 kx125 rear master will bolt up, and can probably be had for cheaper. I used a 99 kx250 rear master on my 85, but did not have a stock rear master cylinder bracket, so I don't know if it's a direct bolt on (looks like it should based on the pics I have seen of the stock one). I was easily able to adapt my bracket by drilling 2 new holes in it.
Pretty much any rear master will work if you fab a bracket.
Edit: I should add, a newer model rear master is great because a rebuild kit can easily be found if needed in the future.
84 YTM200E "yamahauler"
85 YTM200
85 KXT250 Tecate