Some of our customers are surprised when they open their themostat housing and find an original thermosta that looks like this:

Possibly with paper bellows! 
This style of thermostat  typically replaces the old thermostat with a modern thermostat like this one:
180 degree Thermostat

And the spring:
Thermostat Spring


This combination will update your Allis Tractor to a more modern and replacable thermostat.