Hi silverfox - 50 miles might be a bit of an ambitious goal, the problem as always being that there isn't much room on a motorcycle for batteries ... correction:
affordable batteries.
One place I'd encourage you to spend some time if you haven't already is in the EV Album's motorcycle section. It'll give you an idea of specs, performance, range, cost, from people who have already built their own:
EVAlbum: Motorcycles
Also, we've got two resident electric motorcycle owners on the forum. Hopefully they'll pop in.