Allis made a One-Seventy (spelled out), later changed to a 170 and that later became a 175 (both sold for a short while). Think all 3 of those should have the same fuel tank, gas or diesel. This was about 55-62 HP tractor made from 67 thru about 1979?
Allis also started making the "7000" series in the early 70's. These were 100HP+ tractors. First models were 7030/7050 which later became the 7040/7060. 7040 later became the 7045. 7000, 7010, 7020 and 7080 were other models added after over the years. 7000 was a very different tractor underneath the sheet metal than all the rest of the other of this series. If he can give you the specific model it should be pretty easy to find fuel tanks in a good tractor salvage yard.