will rc51 forks fit a 2001 bird?
if so what kind of mods need done

Yes they will. You'll need the front brakes, wheel, upper and lower tripple clamps, the front brake master cylinder, front fender, and de-link the rear brakes.

Obviously, like demon said, you'll need the parts, but unfortunately, it's not just a direct bolt-on swap.

First you need to turn the steering stem down for the lower headstock bearing as the blackbird's is smaller (or you can make and press in a new one).
Then the lower stops need to be removed and replaced further out. The ignition mount will need to be opened up (the carbed version will bolt right in). Also, the ram air tubes can interfere and will need to be modified (melted) in order to clear.
Then there's finding handlebars that work and still look decent.............good luck with that one. Let me know if you find some.

As Jason said, it's a pretty complicated swap. I haven't done the swap and didn't know about the ram air tubes or ignition. Wouldn't really be worth it in my opinion, but then again it's my opinion.