I have one of these now. It is indeed the same outside dimensions and mounting as the 53,63,70. Have not run it yet.
The main surprise is that the carburetor throttle barrel now has no tapered metering slot at all. There is simply a new tiny hole drilled into the intake side. There is an evolution here from fancy multiport carb. to tapered slot only to now just a hole. This reminds me of the old days two stroke air bleed carbs.
Different parts are crankcase and cylinder and rod for stroking. Smaller diameter crank pin and rod end. And different throttle barrel and carb. casting.
Will run when the weather gets better.