Welcome to the forums!
If you are looking for parts, you might check the classifieds and see what is "out there". Also look on ebay or craigslist in your area.
I do not know if a 1977 had a gray water tank [what I assume is your brown water tank need?] this is for dish water, etc. Not toilet. The holding tank for the toilet is normally referred to as "black" water tank. If you want a replacement, you might see if the one you have can be repaired, otherwise check Vintage Trailer Supply - Vintage travel trailer parts and supplies!
they have lots of stuff! I think you might find a door lock as well. You can also check Airstream Innovations at Inland RV Center, Inc. (951) 734-8130
Depending on what you want for a bathroom sink, lots of folks upgrade their trailers with stainless bar sinks [they are small and work well, and can be found at most home handyman type stores, Lowes, Home Depot, etc.] Wait with the exterior cleaning and polishing until the inside and safety stuff is done.
My husband completely restored our little trailer, but has lots of info on his thread, here is a link if you want to browse. http://www.airforums.com/forums/f36/...mbi-17925.html
You might want to specifically ask your questions for more responses. Such as, Where can I find a new gray holding tank? Or where can I get a 1977 door latch? If that is the title of your thread more folks might respond!
Welcome to the world of Airstreaming! It is great. I suggest you check out the forum rally threads and see if there are any rallies in your neck of the woods... it is a great place to meet other airstreamers and see how the other folks have made repairs, changes, etc. as well as tour newer ones! You usually don't have to have your trailer there, just contact the rally organizer.
Mrs. NorCal Bambi (traveling in S Tardis)