I have made single piece adapters with cranks like that, you just have to take the key out and run a taper when machining the adapter and have internal threads to capture the stud sticking out and continue them through for the prop bolt. When I do that I machine the taper a little shallow so it is more of a press fit, and it hits the outside edge first and not the inside edge. It would take some careful measurements to get that one right though...

I have my McCulloch 32 machined that way along with my Pioneer 52cc, the Mac has a few gallons through it while the Pioneer only has a couple tanks.