Frank, that bike looks to be the James ML, a model built initially for the military, used by Airborne Forces and continued post war in civilian trim. Girder type forks with pressed steel blades, bolted on chainstays, hand change gear lever gate attached to underside of fuel tank. Superseded by the similar James Comet in 1949
If I'm right, you need a Villiers 9D engine, 122cc. Twin exhaust pipes to a silencer box running across the frame in front of the motor....and a lot of other bits.
Swarfy.