My Mum has Crohns. She's running at the moment, although her stomach problems don't affect her normal ability to run. She can get tired when going through 'a bad patch' but is generally ok.
She's had Crohns for about 15+ years & has been running on-off throughout.
Again, I would suggest contacting the NACC - Mum has been a member of them for years.
I got my name in their mag after raising money for them in FLM 2005. I found that, as a small charity who don't get places, they were really appreciative (much more personal than a large charity).