either get a reman starter and mount gasket for it or take it to a starter rebuild shop and have it rebuilt.
Make sure you do replace the mounting gasket.

this is a generic parts lookup:

3001023 R - reman starter gas/lpg
178470 - gasket

If these are not correct then we will need a truck serial number to insure the correct part numbers.

If you want to make sure there are no screwups just take it and have it rebuilt at a starter shop.

The starter is supposed to have a seal. You can't see it unless you take it apart. It is mounted on the inside part of the nose. Also the solenoid should have a rubber gasket where it mounts on the starter.