A LARGE flat washer works as well or better than the official factory wrench. Finding the right washer is the trick.
The ones I have scrounged up that work (for the .76) are 2" OD, and stainless steel. The zinc plated steel washers are much thicker, and didn't fit.
You can apply a large Crescent wrench (alien copies are not suitable!) to the washer, while you hold the engine between your knees. Push down on the washer, and remember "lefty loosie". No, I haven't actually removed the backplate of any of my Stalkers. Alan asked me to do some R&D on what would work, because sometimes engines came in from Ukraine without the wrench.

Steve