If the car is otherwise totally stock (no radio), you just switch the connections at the ammeter, coil and battery. Everything else works equally well with either polarity.
You'll also need to re-polarize the generator. There are different ways to do this, the best IMO is to take the yellow/green wire loose from the 'F' terminal on the control box, and momentarily touch it to A or A1 terminal. The easiest is to just jam your pocketknife (or similar instrument) in between the F and A terminals. That causes more sparks, but saves taking the wire loose and putting it back. Either procedure is done with the battery connected and the engine off.
Things get more complicated if you have a radio, electronic ignition or electric fuel pump. Some of them can be switched or rewired for negative ground; many cannot and have to be replaced with the proper version.