Having gone this route let me add my 2 cents.
First, the correct engine service manual is free on my website. For all who need a manual for the Yanmar GM series, download it directly from this page.
Yanmar Marine GM service Manual
Sorry I didn't see this thread sooner, or I could have saved you some trouble.
As for parts...I live in central Florida, and can't speak for Louisiana Yanmar dealers. However I've found a Yanmar dealer here who is always 20% lower on new parts than what you see online.
Lastly, this same Yanmar dealer has a building ... not a room ... an entire building ... full of old engines they've pulled over the years. It's amazing in this day and age to see a business not part out their used inventory on ebay, but not all are computer literate.
I found all the parts needed to rebuild a 1GM10 by wandering in and out of isles full of old Yanmar engines. That's the good news, now here's the bad news... these wonderful folks have no idea what's in this building. You have to show up in person and ask, and if they like you , in you go. Their prices are what you are looking for.
Different State but not that far. Maybe this will help...maybe not...