Well it took a couple weeks but I finally got that washer from bikebandit. I fitted it and assembled the rest of my clutch. Everything turns freely so I think that may have been the cause of issues. The engine is still on my workbench as I have a plenty of things left to do while I still have it out. I will post when it is finally in the bike and running again.
On a related note, about 1 minute after fitting my clutch I reached for my kick starter to install and found washer on it that looked a lot like the one I was missing and had to buy. This washer was supposed to be on the kick starter but it got me thinking. I dug out the kick starter from donor engine and found 2 identical washers on it. Presumably the mystery of my missing clutch washer is solved now, which makes me feel a little bit better as I usually do a good job of putting everything in labeled zip lock bags. I guess this time I still had the part, but I had it in the wrong bag.