Try "Lacquer Stik" from Brownell's.

This is paint in a stick form.
Just use a knife to scrap off the end to expose fresh material and rub it into the stamps.
Use paper towel slightly dampened with lacquer thinner to remove the excess from the surface, leaving the stamps filled.
Let dry 30 minutes or so.

This is very durable and lasts a long time.
Armorers and gunsmiths use it to fill sight markings.

People have also used crayons, but that melts with heat.

LACQUER-STIK(R) | Brownells