Not sure what type manifold heater the tractor you are looking at but Perkins and their Caterpillar twins, 4 liter 4 cyl and 6 liter 6 cyl, had a manifold heater option. It consisted of a heating coil and heat operated valve on the end of a fuel line from the lift pump to the intake manifold. Activating the heater warmed the heating coil which opened the spray valve. Cranking the engine then lit off the fuel spray in the naïf old as it hit the hot coil. The spray was limited so it didn't burn up all the oxygen in the manifold which would defeat the purpose. The heat stayed on as long as you cranked the engine. It was very effective down to about 15 below F. They dropped the manifold heater in favor of glow plugs when they went to a common rail injection system. The manifold heater didn't draw as much power as glow plugs and some were switched so an operator could continue using manifold heat after the engine started but was still running rough. Ether is not good with the manifold heater because of the explosion that occurs when the ether hits the hot coil in the manifold. Not sure if this is the heater type you are looking at but it was a very common design in the ast although I don't know of anyone using the design now.
