Someone in the UK actually managed to get an Electra-X engine and gearbox into a WDCO (or similar) frame complete with girder forks and fit a replica(?) military sidecar a few years ago, might be worth an internet search to see if you can find out more.
+1 on totally avoiding repro girder forks of unknown quality.
What you CAN do more easily is give the existing forks a bit more of a classic (early 50's) look by replacing the casquette headlamp with the fork yokes/triple tree (steering stem) from a 2003/4 Indian market RE 350 AVL Thunderbird (don't forget the headlamp brackets to match, which you can use with the existing fork shrouds) and repro 50's Lucas 7" headlamp with the old style light switch and ammeter fitted. Careful choice of a repro speedo+mount and handlebar switch gear will complete the front end.
This is the sort of thing I'm talking about, I think the AVL Military has full length tin shrouds, not gaiters, but otherwise imagine this painted army green (and with a 19" wheel, not a 21").
There are various 50's style rear number/licence plates around that will take a Lucas 525 or Miller tail light.
While I can understand your hesitation over using ebay that's probably still best for the Thunderbird parts, Hitchcocks' in the UK list them but they're currently out of stock there.
http://accessories.hitchcocksmotorcycles.com/accessory-shop/front-forks/18923Meanwhile any classic Brit bike spares emporium should have the rest.
A.