Probably no, unless you also convert the truck to 12 volts.
If you run a 6-volt motor on 12 volts it turns too fast, and may not last as long as it would on 6 because the mechanical parts are subjected to more stress.
If you run a 12-volt motor on 6 volts, it may not turn at all, but if it does it will be slow and will be drawing more current than it was designed for and likely will fail quickly because the electrical parts are pushed beyond design limits.
Its possible to rewind a motor to use a different voltage, but those motors are relatively cheap, so buying a replacement motor might be the best bet.
