Well, I'm not a pro, but I have some ideas. First, a hot water tank that old is just asking for other problems once you start working on it, so my preference would be to replace it. But, if what they suspect is wrong is in fact the only thing, you might be able to buy some time by replacing it. There are, or at least should be, a dielectric coupling that bridges the connection between the copper house piping and the steel hot water tank. This coupling basically has an insulator in it to prevent the two different metals from touching. If they do, a small current is generated between them (actually like a battery), and one side corrodes, usually the steel. Eventually, this will become weak and leak. In the process, the iron in the union can create nasty rust and block the pipe somewhat or nearly completely, as in apparantly your case. So, depending on how the connection is made (rigid pipe or a flexible metal hose), it may be fairly easy to disconnect the existing one and replace it with a new one. This could all be just unscrew the old one, and replace with new parts, or it could require some soldering. It could come apart in pieces as you take it apart, too. But, my guess is that you may end up with a leak. The parts aren't expensive, and you might get a little more time out of the tank, but replacement is in your near future. My unprofessional opinion.