Heat sensor---do you mean the one on the radiator for the auxiliary fan, or the one on th ehead for the temp gauge, or the one on the head for the fuel injection?

If the part with the cracks around the mounting holes is the black fiberglass piece just in front of the top of the radiator, it's easy enough to replace if you can find a good used one. Should be the same on 90-92. May be the same as later 940 models. You might be able to repair/reinforce it with JB weld or a similar epoxy.

Sitting with no coolant and low oil should not be a problem. Hopefully the system wasn't open to the air, but even if it was it probably didn't hurt anything. Filling the cooling system is relatively easy, as the cap is at the highest point. Fill the system by the tank on the right inner fender. Warm the car up with the heater on, until the thermostat opens. Allow to cool and top up as needed. Use distilled water to avoid deposits or corrosion.

The thermostat should have a gasket. It is a black rubber ring with a slot on its inner edge. The rim of the thermostat fits into the slot. You shouldn't need any kind of sealant if the surface of the head and inside the cover are clean and reasonably smooth.