This is a very simple fix. There is an interlock between the foot brake and the shifting mechanism, when pressing the foot pedal the brass pin retracts from the cams on the shift lever. You need to take apart the cowling to gain access to the interlock pin at the shift lever. Behind the pin you will see a lock nut which needs to be loosened, you may then adjust the pin set screw to the point where it functions properly or back it out far enough so that it does not function at all (my choice).
Good luck.