1. Drain the reservoir, then repair any cracks and put waterproofing (they make a trowel-on concrete like spackling) on the inside surface.
2. Install a flexible sheet plastic liner or bladder (may or may not require draining the reservoir.
3. Don't drain the reservoir but have a not-so-deep sea diver go down and patch any cracks with hydraulic cement.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.