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If you have replaced the thermostat, the most likely problem will be the gauge, the sender, or the voltage stabiliser. (or a wiring fault)
First thing would be to confirm that it really is warming up. This can be done by measuring the engine temperature with a remote reading thermometer, or the temperature of the header tank with any thermometer (caution opening the cap when hot).
If it is not getting hot, then it has to be a faulty (or wrong) thermostat or faulty installation, although I am at a loss to see how this could happen.
If it is getting hot, but not showing on the gauge, the voltage stabiliser is easily discounted - if the fuel gauge is working correctly, the stabiliser is working correctly.
If not, or there is some doubt (i.e. there may be a separate sender fault for the fuel), disconnect the wire from the temperature sender - the one screwed into the side of the thermostat housing, not the triangular one, which operates the choke light. Then measure the voltage between this wire and earth (ignition on). If the voltage is fluctuating between zero and about 12.5 on a cycle of a second or two, then the regulator is working, although not perhaps correctly.
This also confirms that wiring is intact.
What you have not made entirely clear is whether the gauge actually moves at all. Assuming it does, and the fuel gauge is reading correctly, and the check above shows wiring is OK, then I would be inclined to order a new sender (it is unlikely that the gauge is faulty).
If both fuel and temperature are under-reading, then a faulty stabiliser is the problem. This is a small metal box on the back of the speedo.. If it is working but not correctly, then it can be adjusted - there is a small shaft ending in a flattened bit sticking out of it. This can be turned and the output adjusted to give full fuel when the tank is full - care with the lag. All wires including earth must be connected for it to work. Or yoou can just replace it, perhaps with a solid state one.
Hope this helps.
John
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