As long as they are the same thread size, the saito should work on the O.S. The only issue may be the washer as some of the saitos I have, have a metric prop shaft and imperial thread size. Or, look at the equivlant size Magnum for a replacement washer/nut set. The one for the Magnum 52 four stroke is only like $6.95 for the three piece set. 1/4-28 thread size.

You are correct about the washer thickness. It is a much more robust washer than you will ever find at even a well stocked hardware store. The hardware store washers are stamped from sheet. The thick tapered prop washers are machined to shape and size.

Don