I refreshed those lines on a Unertl using artist oil paint (white lead) mixed with enough turpentine so it would wick into the scribed lines from a paint brush. Used a very small artist's paint brush with about 6 hairs. It worked great and being oil paint will last for years & years.
It takes some time but use a magnifier to aid in placing the brush on each line and the paint will flow to fill the line.
The day after you fill the lines, go back with a Q-tip moistened with turp to wipe away any white paint that found its way outside the lines. This one of those situations where you want to stay inside the lines

I've used whiteout and erasable markers.  paint, count to 3 then wipe. Wait a day then over coat with lightly thinned clear matte nail polish.