There is no need to put a thermostat on your fattywagons heater. As long as you are running fuel through the line, the heater won't get too hot. When I first installed my heater, I could only get 65ºC at the injector, so I added insulation to keep the heat loss to a minimum.
On my Changfa, I'm powering my fattywagons heater it with unregulated a/c (~16vac) from the battery charging alternator. I am getting from 80º to 100ºC at the injector depending upon the load.
On my Metro Listeroid, my Fattywagons heater is powered by regulated 14 VDC, and I am seeing at most 80ºC at the injector. I also insulate the injector line to keep heat loss to a minimum.