KTM, YZ, WR will all work with very little modification. Only the steering stem and yokes need modifying; the lock stops and brake caliper mount may need fiddling with. CR and KLX forks can be made to work too. If you want to keep the costs down then KLX(KYB) forks seem to be the cheapest to get hold of secondhand.

Here's one thread showing a fork conversion on a budget (although some images are missing) worth a look and will give you an idea of what's involved:

http://www.xt660.com/showthread.php?t=20845

Here's a couple more:

http://www.xt660.com/showthread.php?t=20031

http://www.xt660.com/showthread.php?t=20420

__________________