There are tank liner kits available:
Kreem, and others. Will seal small, pinhole type, leaks.
As far as side covers go, they are hard to find for C's. My father has an '81/'82 Custom. I found a perfect match side cover, off of e-Bay, paid more than I wanted, only to have it stolen last year (bike was not ridden, not sure how/when it was stolen, but it was...). That is how commonly they fall off/are stolen. It is usually the left side cover, that disappears.
Maier Mfg. makes side covers for
some CB750 models, but not the Customs. Sorry.
http://www.maier-mfg.com/catalog/street/honda-0Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII