First of all, its not a Palmer Johnson. Palmer Johnson is a high quality a mid-western boat builder/importer/boatyard. If its and engine it was a 'Palmer Brothers' or more simply a 'Palmer' engine. Palmer was a Connecticut marine engine builder and one of the first marine engine builders in the country, if not the world. That may help your search.
Palmers were simple, robust, easy to work on, but they have been out of production for something like 40 years and near the end were no where near as popular as Universal Engines. By 1974 it is pretty unlikely that a Palmer was installed in a west coast built boat. For much of its history, Palmer made many of their blocks and internal parts. That said, my family owned a 1963, Dutch built boat with an 8 hp Palmer PW27 single cyl engine which had a Wisconsin built block and internals.