Hmm, starting at the beginning the winding system (winding pinion, clutch) would need to be remade to work backwards. The stem would need a left hand thread so custom stem and crown. Click would need to be reversed in function, so some machining there. Barrel arbor remade with reverse hook, barrel modified (if not automatic) for reverse hook. The rest of the gear train is ok, up to the escape wheel. This would need to be inverted. The pallet fork could possibly be inverted depending on design (the guard pin needs to change sides at least), possibly remade. That should do it.