30 PU is basically right, but actually, it's more complex:
In theory, the starter will need 2 times the normal current @ 12 V, as written above.
In reality, an electric motor will try to reach it's no-load-speed. That would only cause a relatively low current and would not be big a problem. You could even operate the starter motor @ 12 V for some time like this without really damaging it.
What limits the speed of a DC electric motor is the induced voltage which works against the operating voltage. So if you take a motor with absolutely no friction that has reached it's no-load-speed, the induced voltage is just as high as the operating voltage, it cancels it out. ---> no further acceleration
Because of the load which the electric motor has to drive (compression and fricion of engine) it will
not reach it's no-load-speed. It rotates slightly under that speed, and if you do not change the operating voltage, it keeps getting slower and draws more current the more load is applied to it.
I'd assume the engine is (at the rotational speed of the starter motor) a fairly linear load, which naturally increases with the rotational speed of the starter.
So yes, the starter will draw a lot more power, maybe not exactly 4 times the normal power, but certainly a lot more than under 6 Volts. I'd guess about 3 to 3,5 times as much.
That's not a big problem for the windings of the motor, the time needed to start a healthy car is not long enough to damage them because of excessive heat. The problem is the collector and the brushes. With the way higher current, the wear on them will be a lot worse than under normal circumstances. So if a starter normally lasts say 10 years, it will not even last half as long with the increased voltage.
That's the reason I'd always either stick with the stock voltage (which works absolutely fine if you have a healthy electrical system) or convert to 12 V and use the correct 12 V parts. With these old cars, it's not even much that has to be changed, so I'd personally not use the original parts with 12 V.
Please bear in mind that the above is only a really simple breakdown of the electric DC motor. For example it doesn't cover the timing of the motor (turning the brushes slightly against the direction of the rotation to increase the performance) or having different numbers of windings or poles.
tl,dr: Use the proper motor for your voltage.
