John,
The reason the plastic "thrust washer" looks eaten away is that it is not
intended to be a thrust washer.
Neither it or the strut are intended or designed to take thrust loads.
Your best bet is to take it to a yard that knows their stuff, and see what they
recommend. That or a marine engine specialist. They will be able to recommend
any of a number of aftermarket thrust bearing setups that can be installed, if
somewhat painfully....

Steve