It's been in the ultrasonic cleaner for several hours over the weekend, and now looks like this. Still a work in progress.
The rust dissolver solution I am using is breaking it down, but there is sticky old gas residue in there too, which needs scraping off. However, the electric contacts are now clear of rust and I can confirm there is a good circuit to the low fuel lamps and the fuel pump, which is progress. Before cleaning there were a solid mess of rust and were probably shorting out on each other due to that.
Not finished yet, but happy with what I have achieved so far.