Try Evinrude or Johnson parts
The OMC Zephyr SailDrive was basically a de-tuned 35 hp Evinrude/Johnson outboard powerhead. The Zephyr was 15 hp, but most internal parts are the same as the 20-35hp Evinrude/Johnson outboards of the time. So, you could also buy the powerhead for one of those outboards, I see them on eBay quite often, you could buy the whole outboard and sell what you don't use to make up part of the cost.