The bigger Yam,OMC, Mercury use the same pattern.
I use a hardwood dowel.
If the holes are in good shape, you can just fill them with out any prep.
I usually re-drill to the next larger size so the edges are crisp/clean.
Any moisture at all, you need to dry the transom,.can take weeks and weeks??
I cut and pre-fit the dowel to be just undersized by 1/8" on both sides of the hole(if both sides are visible??)
Then fiberglass resin(make it hot for a quick dry) put it in the hole and on the dowel.\
Insert and wipe off excess and let it dry.
Then sand smooth.
Then a small jar of Marine-Tex to fill the hole and let it dry, sand/smooth.
I use a sandwich baggie cut in half or plastic film and taped over the patch.
Makes it dry quicker.
Take pics and post as you proceed.