Regarding store bought washers, if you go for the ones for automotive, they are usually Grade 8, which is about as hard as you're going to get. For the smaller engines, I've found the standard hardware lower grade sufficiently strong, plus they are easier to drill. Vintage engines I have, parts are sometimes hard to come by and when they are available, seems some folks want more than for what they are worth. I'm less concerned about authenticity and more concerned with functions.
Glad to see you were able to find what you were looking for without having to pay more for a set with other part you don't need.