Look at the parts in Section 3.

You have several options
Just the seal and pushnut Items 417 and 415
A seal & impeller kit Item 434
Or the pump assembly.

I would pull the pump apart before deciding what needs replacing.
If the motor shaft is rusted you will nee a new pump.
Could be it is time for a new unit.

First I would get in there with flashlight and a small mirror, to try and confirm that it is the seal that is leaking.
It could actually be leaking higher up.
Check the clamp Item 457.