Motocross tires for the X

I want to put 21 inch front knobbies 18 inch rear knobbies on the x. i know there is a thread on here about buying a set from forest at wheeling cycle supply. i had the crazy idea of building my own. how do i find the size of hubs/spokes/rims ect. does anyone have any of this information?
thanks in advance.

I have an X set of wheels for my R. They use different hubs and therefore the rotors are not interchangeable. Not to mention 2 different sizes. I also have 2 complete front brake sets...1 for the X and the other for the R. If you're going to build your own, you should probably start with some R hubs and lace them up to something like an Excel wheel. However, I think you'll have to have spokes made by Buchanan if you go with an Excel rim. A guy over on ADV had that issue. Then you'll need to find yourself an R caliper in order for you to use a smaller front disc. Which then means, you'll want another front master cylinder and brake line to make the swap easier. Un bolt one set up and bolt on the other one. Do yourself a favor, unless you find an entire set up from an R (wheels, caliper, master cylinder, rotor) just buy them from Wheeling. It might be more coin than you want to spend. But at the end of the day, it will be so much easier.

http://www.buchananspokes.net/

If you can lace and true call buchanan,if not forest. Its not a crazy idea as i did it. Just tell them what hubs you want to use. The yamahs rim is pricey,have buchanan drill one. Check yamaha motor sports plaza,better prices.
You can score an early suzki 21 and save money,but you will need to swap brake caliper (kx works) i have done this also.
http://www.supermotojunkie.com/showthread.php?95982-cheep-21-inch-wheel&highlight=cheep+21