Our local dealer has priced his Yanmar parts, filters in particular, at prices where I wouldn't consider using the wrong one. You might check around and find that, except for the big-box stores, most filters will not be much cheaper anyway. Compare the price of a filter every 6 months or a year to the cost of operating the boat and it becomes a noise-level problem.
It is not quite so critical on a fuel filter but there are some tricky things in oil filters that might be risky to assume. Bypass settings, drain valving, etc. can vary enough that they may perform fine for some applications but won't give ultimate life in all applications.
Fuel filters, as long as you make sure the rating in particle size is at least as small as for the Yanmar, are interchangable. But where do you get that particle size? Surely not from Yanmar which tends to play it's cards pretty close to it's vest these days!
You can buy belts at NAPA if you choose. Generally speaking, a belt is a belt is a belt. But finding an exact replacement seems to be tricky there, too. The Yanmar engineers seem pretty good at shaving the dimensions so a precise fit is hard to find. But with a little shopping, a friendly supplier who will let you try a few, and sweat and skinned knuckles while you try them, you can find a belt someplace that will do just fine. As laid out there, is that worth it either?
Obviously it might pay to do some comparison shopping at a few Yanmar dealers and service shops in the area.
Fluids can be bought anyplace, regardless of what the Yanmar manuals say (and the warranties try to scare you about) as long as the specs match. If the specs from reliable ratings (SAE, API, ASTM, for instance) match, the product matches. It may be best not to mix products due to different formulations, but if the specs match, performance matches.
