Gunsmiths and armorers use "Lacquer-Stik" from Brownell's:
www.brownells.com/aspx/NS/store/productdetail.aspx?p=7778&s=This is actually paint in a stick form.
To use:
Use lacquer thinner and a brush to degrease the lettering, allow to dry.
Scrape the end of the stick to expose fresh material, and rub the stick into the lettering until it's filled level.
Allow to stand for a few minutes, then use a drop or two of thinner or a little mineral oil on a rag or paper towel to remove the excess.
Allow to dry for 30 minutes or so, then apply a coat of lube to prevent rust.
This stuff is very durable and is more or less permanent.
It can be easily removed with lacquer thinner and a brush.
There are other products that will work, including White-Out, crayons, model airplane paint, etc.
Be warned: Some products like White-Out can absorb moisture. Remove the White-Out and you can find the lettering is rusted.