First, check the gas inlet screen, sometime they get clogged with debris and that can increase the pressure drop.

Also, I know gas lines are expensive to have installed but isn't still cheaper to replace the line than buy a new heater? In the end, it may extend the life of the heater as well.

But if you decide to replace the heater, the size of the gas line required is dependent on the size of the heater so if you go with a smaller heater, your might be able to get away with the line you have. It might even be possible to have the heater modified so that it uses one less burner (reduced BTU) but that will increase your heat time.

I still think your best option would be to consider an new gas line.