Taking the tank off and resoldering it is the easy part. Finding all the corrosion and repairing it is the hard part. Getting a good fit is also important. A slip fit is desired... any looser than that and the solder has to much of a void to fill. Capillary action only goes so far.

If its a tube and fin core I would attempt it if the fit and metal are good.

If its a stamped/honeycomb core then no.. you will need a big ol tub of molten solder.