The 54 and 60 cranks are threaded 3/8s instead of 8mm like the smaller engines. The taper is also slightly larger. You can saw out the center portion of the stock flywheel, turn it the same diameter of your prop adapter. Install the magnet in this.
I'm up to my neck in motors now No new projects planned unless I just stumble across something that looks promising. I do still want a twin though.