It was easier just to plug the drain. Happily a 15mm push fit stop cap was the correct outside diameter:
All the major functions work, it fills, it sprays, it heats.
Minor setbacks are that the head of water isn't enough to make the pressure switch function so it'll have to be manual fill. A push and hold switch for that for flood prevention. Only the arm with the bigger holes spins by water pressure, the others move by hand so I might have to reduce the flow to the lowest arm. That can be tweaked when it's built. The current heater has no place to add a thermostat, improvisation will be done, or the other heater and tube will be refitted.
The next job is wiring and plumbing it neatly.