Hi Cat and welcome to the forum,
it's encouraging to hear that you are trying to start a new event as so many are disappearing across the country.
I would suggest you contact the National Traction Engine Trust
www.ntet.co.uk as they help the organisers of a good many rallies and will offer you advise on the steam side of things.
Here and the MOOS
http://mechanicalorganownerssociety.weebly.com are the places to contact organ owners.
The National Vintage Tractor and Engine Club
http://www.nvtec.co.uk/ is a good place to start for stationary engines and tractors.
As to steam powered organs that's a a bit of a misconception, apart from the very rare Calliope, fairground organs are electrically powered, though the smaller ones can be hand cranked. Originally the electrical power came from the generator on a showman's traction engine or sometimes a dedicated steam engine in a separate trailer. This arrangement is recreated at many rallies when the owners of engines and organs get together, but the reality is that most organs spend most of their time powered by a diesel generator.
I wish you luck, but be prepared for a lot of hard work as organising an event in these health & safety and red tape obsessed times is not easy.
Regards
Greg