I have just been doing a search on the Forum on your behalf. However there seem to be more promises of X craft plans than actual plans!
To help give you some pointers (and I see that you live in the US.):
- Original X craft are on display at both the RN Submarine Museum, Gosport and the Imperial War Museum at Duxford. This might not seem to be very helpful information when you live in the US but if you go on to their websites you will see photos/ walk rounds etc and they can be contacted for photo archives etc.
- The Dock Museum at Barrow in Furness http://www.dockmuseum.org.uk/ have the Vickers Photo Archive. (Most of them were built at Vickers, Barrow in Furness. In fact one of the members of my model boat club (Barrow) worked on one of the later X craft.)
- John Lambert did do a plan:
X51 Class Midget Submarines
Model Boat Plan: MAR2096
But following his death last year I don't know about availability. His daughters are cataloguing his plans at the moment. (See the thread on this Forum and let us know if you find anything more.)
- There is some info in my favourite "fall-back" book. "Observer's directory of RN Submarines 1901-1982" by M P Cocker publ by Frederick Warne (1982) Some nice photos, specifications and some (very small!) drawings. Let me know if you are having problems finding a copy.
- The photo archive on our new website shows some good models:
Please let us know on here how you get on and what you find. There does seem to be a shortage of information but there are some excellent models out there (Nigel Edmonds and Alan Poole come to mind immediately) so there must be plenty of information.