Living in Charlotte I would NOT recommend converting a 123 car to r134a. You will not have adequate cooling capacity for the summer.

I would not waste your precious R12(I've heard as high as $80 per pound for it now) without fixing the leak first. Usually the most difficult part of fixing a leak is finding it first. Replace the hose or whatever you need to do, then evacuate and recharge. Don't forget to replace the filter/drier this time.

Good luck,