Time to Fix It Right

Start with a compression test and oil pressure test to see if the engine in in sound condition. I'm assuming it doesn't make and out of the ordinary noises? Once you establish that the engine is in good shape internally, work on the carb and ignition systems. Your symptom that it runs worse with the headlights (or other current draw) on is troubling. I'd take the bulkhead connector apart at the firewall and check for corrosion to start. Next would be a charging system test and voltage reading at various points to find any bad voltage drops.