You should be able to find info on the web like this...
http://www.carolinamcevents.com/pokerun.html
which explains poker runs (which can be set up for charity)

Or write to some of the different charites that already run annual events - ask for pointers, info, what to watch out for, ... http://www.travelingbikers.com/charities.html

Or check around your home town/county/parish.  See what info you can gather from local events - and in doing so, set up new contacts who might join forces with you or pass the word around when your ready to do it.

And contact your local motorcycle dealerships - they might be able to help too.