I would love to do one of these myself, and have searched for info on how to make it happen as cost effectively as possible. The best source of info I found was So Cal XR's. They have a good article on their tech page on doing a conversion, and they also offer frame mods for less than some. Nothing quite as trick as the BBR setup, but I just can't afford that no matter how much I'd love to have one.
I'm hoping now that Yamaha releases it's YZ125F, or whatever it will be, about the time my 9 year old will be the right size for it. That way, I'll be able to get him a really cool bike, and I might get to play on it occasionally myself.
Good luck with your project.
By the way, the web site for So Cal XR's is either socalxr.com or socalxrs.com (I think)