Welcome! We have resources all in one central place.
At the website, login. On the left column at the bottom is WIKI. Or if you are using Chrome, it's weirdly located in a batch of icons at the bottom of the screen somewhere. Click on WIKI. Next locate the RESOURCE PAGE and click there. It's a big list of dealers, parts suppliers, salvage yards, the FREE manual section, and other goodies.
As you have worked at the prior dealer, I know the community here would be all ears. Why? Documentation is hard to find, even accessories for the machines alone with other options. For example, we now have the snowblower parts manual for 2 of the sized units, but not the other 2. And like yourself, with your dad's machine passed down to the next generation, it would really help others out that are in the same situation.
The YM276 is a great machine. Next to the YM240 and the YM336, it's a sought after machine. From some of the known brochures, we know the YM276 is related to the YM226. Also, the YM276 gives many clues with the YM gray market machines with the 3T84 engine. One of the most widely made engine series in the entire YM models. So, obtaining parts should be better than a few of the 2-cly engines with limited models for production.
Again, welcome to the Yanmar Tractor Owners group. Our history goes back some 35-ish years, GeoCities->Yahoo->GroupsIO. It's possible your dad had interacted or found helpful info with the group in those years. :)