Carl,
Try Villiers Services, West Midlands, UK. Tel. 441384 265797
or Fax 441384 76352. No website or email, I'm afraid.
Or
www.nortonvilliers.com Tel. 44161 790 5277.
Your Welbike should have a 'tuned' Villiers Junior Deluxe engine, many of which powered (I use the term lightly!) a multitude of lightweight motorcycles before, during and after WW2, so parts should not be that difficult to find. If you let me know what you need, I may be able to source them elsewhere.
I'm glad to know another one of these very rare machines is being restored. A poke in the eye to all the poseurs who try to pass off khaki painted civilian Corgis as wartime parabikes!
Regards,
Rhett.