This old hammer lost one of its striking surfaces.
This project prints a mold and stem base for a silicone rubber replacement.

I used an RTV silicone rubber (Tinsil 70-60 Polytek) to pour the replacement hammer head.

http://www.polytek.com/index.php?dispatch=categories.view&category_id=228

This is a high tensile strength, dense rubber.
The mold is about 25 ml in volume.

Designed in trueSapce and printed on an Up!3D in ABS from Octave.

Print out the two parts.

Mix up a small batch of High Tensile strength RTV (room temperature vulcanization) silicone rubber and pour the head.

Don't worry about a release agent, nothing sticks to silicone.

Place the base piece in to form the inner stem opening for the hammer.

Place a small flat piece of wood down the side of the cured mold and walk the part out slowly.

I used Goop as the glue but E600 would also work.