Attaching a "fish" zinc to the shaft or engine
is unlikely to provide any protection for the strut. The zinc must have direct electrical
contact with the item in question to provide protection. The strut is probably not in direct electical contact with either the engine
or the shaft. You could (as previously mentioned) attach it to the strut bolts, but what a PITA, IMHO.
Most likely, the current
strut zinc has become passivated and/or has poor contact with the strut. I suggest cleaning
the strut very well and installing a new zinc.