As long as youre rotating an armature (basically a coil of wire) inside a magnetic field a DC Generator still produces voltage REGARDLESS which direction the armature is rotating. Youre still rotating a coil of wire inside a magnetic field thats North on one end and South on the other...BE SURE TO RE POLARIZE THE GENNY PRIOR TO START UP HOWEVER because until the field curent flows (creating the stronger electromagnetic field) the residual magnetic portion of the fields determines charging current.
(A) If the brushes that ride on the commutator wear in to one angle and you change rotation of the armature there could be some problems until things get worn in and accustomed to one particular direction. If all is new and the brushes are still flat and theres not too much play then it wouldnt matter so much which initial direction the armature rotated.
(B) If theres a fan on the pulley end it either pushes air from front of engine out towards the rear or vice versa, while it makes some difference since air is different temp up by radiator towards engine rear, I dont think there would be too much, if any, heating problems BUT NO WARRANTY ON THAT Im an electrical NOT mechanical kinda guy..
NOW IF YOURE TALKING ABOUT REVERSING DIRECTION OF A DC MOTOR I.E. IF YOURE APLYING DC VOLTAGE INTO IT AND WANTING IT TO TURN A DIFFERENT DIRECTION things are a bit different BUT YOUR POST SAID GENERATOR NOT MOTOR
If it were a Permanent Magnet Motor and you reverse input polarity it spins the opposite direction,,,,,,,,If its an electromagnetic field motor (like a tractor starter motor) reversed input polarity DOES NOT change direction
REGARDLESS be sure to Polarize the Genny BEFORE starting the tractor