the heater has a very non linear curve, such that It draws quite some current when cold. best solution would be to use a triac driven on input voltage > tbd and use thta the connect to the heater on the AC side of the generator. unless the generator is a 3 phase type, but switching dc powers creates always turn-off surges and sparks. best would be a PWMpower fet control with the heater as a load, so there is no hysteresis and oscillation. allways add surge protectors, snubbers etc.
good luck !