more voltage or amps
The minimum speed of 6v motor to produce electricity is 15 Mph.
6V dc motor
A 6v motor might not run at all on a 4.5v battery. Whether or not it does, it's likely to discharge the battery rather rapidly. A series resistor will help the battery last longer, but will make it even less likely that the motor will run at all. You're better off taking some time to find either a 4.5v motor or a 6v battery.
IC 7806 is the voltage regulator that gives 6V dc voltage from 230V ac power supply. It is used to give power supply to the IC which operates on 6V.
If the machine is designed to run on 6v and you use two 6v batteries instead of one, then . . . -- If you connect the two batteries in parallel, then they will last twice as long between recharges as one battery does. -- If you connect the batteries in series, then they will definitely overheat the motor, and possibly destroy it.
Well first, more pistons means more exploding. That means more power is transfered to the wheels.
Use an "Inverter". They make a lot of inverters for 12v but few for 6v. You could use 2 6v batteries hooked in series though. Just make sure the inverter is big enough to power what you wish. A 300 watt one will power a electric razor, or small electronics fine. A 1000 watt would power a computer or a laptop also, etc.
No, underpowering will not "power" the device, it will not run. You must get an adapter that is 6V and (2A or higher) will be ok as well.
1.25 should have more power but it depends on the design.
MORE POWER
It is: -10v+6v = -4v
Most probably you are using a AC (117V or 220V) to DC 6V converter adapter. If yes, then the answer is no, you cannot use a lower current rating for a device that draws more current. If you plug in a device that needs DC 6V 500 mA to an adapter that can only supply DC 6V 300mA, then the adapter could start sending higher than 6 volts current which may damage your equipment or overheat the adapter. If you are using a higher mA rating adapter, then it's okay. For more technical details, see: http://www.jaycar.com.au/images_uploaded/plugpack.pdf