[PbFut]

I did not know they made fiberglass tanks either. Well easy to add nipples and repair.
As for cleaning, that's easy if not too bad of shape. Some good degreaser detergent should get most out. If the varnish is too tough, not sure. Techron mixed with something? Just test on outside first that it does not soften the resign in the fiberglass.
I think you will like the electric valve set up. It works really well. If you need help with the switch let me know. It's a bit tricky to get your head around the wiring using 4 poles and 3 positions and controlling 2 valves that require the 12V polarity to reverse to activate the throw in the valve motor. But I found it fun to figure out and very simple once you see it.