firstly, just give it a really good service.

Change oil and air filters,
new spark plugs and leads
fresh gearbox oil
new oxygen sensor

What does the temp guage do when youre driving it, does it change at all? If not then it could be that the temp sensor/sender is gone, which is causing the ecu to stay in cold start mode, which will use lots of extra fuel. So change the temp sensors, and change the thermostat while youre at it.

Then either rip that cluster out and sort out the wiring, or get an auto elec to sort it out for you.