I found some info on briggs & stratton motors, here is a link to set the timing.
Briggs Stratton
Of interest it speaks of adjusting the timing for kerosene use by moving the armature bracket all the way to the left and adjusting the armature air gap to 0.25 to 0.36mm
If you need more advance then possible by adjusting the magneto or the armature then you could file out the key slot on the flywheel and make a key to fit.