You need to find a switched power source for the positive post, and you can ground the negative post to one of the carb screws.
Route the power through a three-pronged oil pressure switch. (One prong drives the oil pressure light just like you have now. The other two prongs are an inlet and outlet for the current to the choke coil.) You do NOT want the choke to get power until AFTER the engine has started. So the oil pressure switch interupts the circuit until the engine has oil pressure. Hopefully, the engine has oil pressure because it's running, and not just grinding along on the starter motor...
