As you are in W Yorks you might find a bike or two turning up to the Meltham Memories do in a couple of Saturdays time, but that is just a guess on my part. Might make it myself but it isnt definite at the moment. I'll be glad to chat bikes if you collar me, as will most owners. 'Over the hill' to Saddleworth in August will definitely have few in attendance and some of us will be making the trek to Stanford Leicestershire for the Taverners Founders Day event at the end of July. The theme this year is military motorcycles so if there aren't any there then you might as well give up! :-)
As has been mentioned dont expect a bike to be a cheap alternative to a 4 wheel vehicle but dont be put off. Wartime bikes tend to attract quite a premium over their comparable civilian counterparts. If you get one sorted (preferably by doing as much of the work as you personnally can, that way you know it has been done properly) then you will have a vehicle that will run well, be reliable and do pretty much what you want and above all else be a whole load of fun in its own right irrespective of the re-enacting side of things. In my experience dont expect too much help from collectors of classic and vintage machines. For the most part, in 40 years of biking, I've found they tend to be a bit disdainful of military models so you will find more help on such forums as this and the WDM20 site refered to - a v.useful forum. All the best in your search.