What I'm going to do I'm going to put a step up circuit (5v to 12v) to my rc crawler like this: Battery--->main circuit/receiver--->step up circuit--->DC motor.

The reason I'm doing it is because the 280 motor is rated 12v+ and the battery is only 7.4v (8.4v when fully charged)

Not sure what a step up circuit is. I think I'd maybe look at setting it up for a 3s system. Or maybe a 2s motor.

IMO no
motors generally need quite a bit of current, and to build a step up circuit capable of stepping up the voltage, and supplying enough current if possible(Honestly I don't think it would be in RC size)... you would most likely be cheaper to just replace the electronics with stuff made for either a 2s, or 3s voltage levels.

Also you would then need to design something to control the voltage of the boost circuit as the voltage boost circuits I know of are static voltages... so essentially you would have to make another ESC and have it to pull power from the ESC, and add even more batterys in parallel to give it the current it needs...

So like I said get a 3-4S capable ESC, and 3S batteries, or(cheaper) a 2s motor.