42000 BTU/hr would be about 12,300.3 kW/hr. The best way is to replace the heater with a one or more infra red heaters up to the required amount of heat you need. If you place more heaters you could position them on different spots. Only where required. I would start with 4 units of 1500 W and then see where you arrive.
Please keep in mind that you should seperate the heaters over different groups. This to prevent to problems with your breakers and wiring.
