A very timely question for me, also. Having just removed my head, I'm able to measure what I'm currently using, as well as the diameter of the landing area. First, I believe the answer regarding high tensile is, or at least in my opinion is, "yes." In the U.S. that means high-strength "Grade 8" zinc or cadmium plated steel. The important thing is that the washers do not deform themselves or the alloy when tightend down to high torque (I'm going with 38 ft. lb.). The fact is, I don't know what the stock item looked like, so I found the closest thing I could use from the nearby hardware store. By my measurement, the landing zones at the holes for the head bolts were machined using an 13/16" counterbore. Using my digital caliper, it apperas that their diameter is right around 0.814". I found Grade 8 wiashers that are, quite coincidentally 0.813"-ish in O.D. and 0.060" thick. So, it is a snug fit, but they all fit in the landing zones without distorting, so, I think I'm good. I double-up at each location for a total washer thickness of 0.120" If I knew I could get them, and fast, I would prefer 3/4" O.D. x 1/8" thick. I should point out that part #2-2138 is 3/4" O.D. x 0.064" thick and, as far as I can tell, common steel.
Having said all this, I would like to hear from one (or more) of our experts to help sort it out and advise if my current approach is the right way to go.
Richard L.