Re do them all, 50' nickle copper lines, couple box's of fittings, bubble and double flare tool and a long afternoon and walla.

Piecing in a line you need to find where the line isnt rusted to hell and cut there and run that line to the valve, assuming you can get the valve fitting off without twisting the other line off (vise grips and soak soak soak)

Old to new




Double flare for the threaded coupling/union


If you can find a hand held hydraulic flaring tool get that