You could put a resistor in series with the stater motor. If the resistor and motor have the same resistance then the voltage will be split between the two (i.e. 6V p.d over each). This is assuming that the starter motor has a (roughly) constant resistance in startup. A potential problem is that resistors are normally small (so have a small thermal mass) but there will be quite a large current going through at start up so it will heat up very quickly (and maybe burn out).
Alternatively; if you assignment allows you to be practical/use common sense, go and get a 6V motor bike battery (not as common as 12V but definitely available).
You can make/buy a DC-DC inverter/transformer combo that will step up/down DC voltage to DC voltage but they normally wouldn't be capable of driving a load like a stater motor as they can't handle much current.
By the way if the car is an old 6V system it probably has a positive earthing, while modern car systems are negative earthed.