you can use the old tank, but it will cause you more grief than what it's worth. Even if you "clean it real good", there probably still be rust inside. There are coatings that are available, but they seem to degrade after a time.
If you do decide to use the old fuel tank, put a cheap inline filter next to the gas tank where it is easy to get to, because you will be changing it quite a bit. Then a second filter before the carburetor. Keep at least one, but better to keep two replacement filters under the seat with the tools necessary to change them, because the filter will clog only in the middle of a busy intersection.
