New factory gas tank will cost you a couple hundred bucks.
Wiring is generally very simple and I can walk you through it. That said, keep in mind that people wire in their own stuff, change to "more period correct" items, etc... so it may or may not still match factory wiring. Depending on the year it may have headlight wiring through the turn signal switch or it may have everything wired through a VW style relay.
A car that sat will most likely need the fuel system gone through (easy) and carbs rebuilt/cleaned (easy if you know what you are doing). Carbs can also be sent out to an expert. Cort Drake has a few contacts. I use Art Thraen at ACE if a quick clean and rebuild doesn't do the trick... Art is around $500 for duals, but done right and never an issue in my experience.
#1 priority would be find the fuel "leak". If it is dripping somewhere then it should be easy to find a damp area. If it is running past the needle/seat and overflowing the float bowl then it is going somewhere, like down the cylinders and into the oil. Check the oil for a fuel smell, see if it is thin. Gas is a solvent so DO NOT run the motor on thinned oil/gas filled oil. You'll wash bearings. i know it may sound like a lot, but it is actually a straight forward thing to address.
You can e-mail me the chassis number of the car and I can pull a build file and see if it was a turn key, kit, roller, etc... It can make a difference...
