What gear ratio?
How many cells?
How much does the plane weigh now?
How big is the plane?

Your plane may not have a useful payload of 6oz, no matter what motor you put on it. There may simply be more weight than the wing can lift.

Like I said, just putting a bigger prop on it MAY increase the power. MAYBE. Putting a larger prop on the motor may just cause it to burn up. It depends on how it's configured now. Unfortunately, you only provided half the information necessary to make that determination.

Maximum thrust with good speed can be made by a 6V 380 motor on 8 cells with a 3:1 gearbox and a 10x6 propeller. It may very well be that you have to change the motor, but you have to change EVERYTHING. That was my point. Sorry if I confused you.