There is only one way to fix that fuel overflow and that is to completely disassemble the carb and seal up the needle and seat. Quite often the needle and seat contact surfaces become corroded or fouled by years of build up. You've got to get these surfaces sealing if you want to stop the overflow of gas in you carb. If you don't have clue what to do, here is a step by step.
Disconnect the top cap (where the cables enter the top of the carb) and undo the clamp that holds the carb onto the intake manifold. Disconnect the fuel line at the carb. Remove the carb from the motor.
Your problem is overflowing gas from the float bowl. That is the job of the needle and seat (ie. the pointy end of the pin on the float assembly and the seat within the the inlet cap on the motobecane Gurtner carb). The float mechanism is designed to allow just enough fuel to enter the float bowl and to stop the flow of fuel beyond a specific height. Here is a little tip to get your old carb's float to seat (ie. to shut off the flow of fuel).
When you have your float (with pointy pin) in hand and the corresponding mating seat in the inlet cap, take a little baking powder toothpaste and dip the pointy side of the float pin in the paste. With this light abbrasive on the pin, place the pointy side of the pin in the seat of the inlet cap and twist/spin the pin with your thumb and index finger while applying very slight pressure towards the seat. Do this several times with more paste each time. When you think you've sealed things off, take some spray-type carb cleaner and give the seat a few blasts to clean things out. Reassemble the carb and see if you have it fixed.