I took your picture and enlarged it, and artificially added light to the shadows--
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Best I can tell, if the PO has all the metal parts, you should only need basic rubber hose (designed for marine exhaust) and a few hose clamps. If most of the port side is there, it shouldn't be too tough to match up what you have. It would be a good idea to remove all the port side rubber, to use as sample pieces and while they're off, replace those too...
If you find you're missing a metal elbow or two, try to find an "established" boat repair place, they're likely to have a shed full of used exhaust parts, many of which may work for what you need. For a boat as old as yours, that's what you may be facing...
A friend of mine bought an old boat shop to change into his glass shop, I was amazed at the parts there. Here's just 2 of many pics I took, first one shows some exhaust elbows on the bottom shelf, second was a shed chock full of old mostly OMC parts, much of them exhaust parts... They're out there, you just gotta find em!