I know if you can't find one, take the head to your local bolt store and find one that will thread in then buy a die.
Get a piece brass tubing and DIE the end to the thread size and put a nut on it for tension then solder a brass ferrule on to help hold the tubing. I had to do this on one of my older heads.

Here is a kicker, The water head nipples on the old OPS .60 are the same as the water nipples from the head of a K&B 7.5 outboard...
I was lucky to figure that one out.

Jeff