Does yours have the filter fitted? 54s should have the filter, however they were sometimes changed to the later type as seals for the early ones were hard to find.
ZARwerks make kits for the early ones with filters.
http://zarwerks.com/ A 54 should have the 6mm filter.
Wolfsburg West have the lever & other junk.
http://www.wolfsburgwest.com/cart/DetailsList.cfm?ID=111209167A
Wolfsburg West also have make replacement 56- taps if you want a brand new one that won't leak. (They are interchangeable)
http://www.wolfsburgwest.com/cart/DetailsList.cfm?ID=111209021E
Wolfsburg West also reproduce the numberplate light. Original ones come up for sale on TheSamba fairly regularly.
I'm surprised Dave hasn't popped in with his ring Vintage plug, but they might be worth a phone call to see what they can supply before sourcing them from OS.