The 18 yolks have a different offset and different stem length to the 21, the 28 and 21 yolks should be interchangeable although they are slightly different. The 28 has a hole in the stem for the steering look and the top yolk hasnt got the ignition key holder.

I seem to remember from looking at one of Watfordhorns nc30's the shock is offset to one side so I guess that would complicate things. I believe racer38 fitted a single sider to a mc18 many years ago so it must be possible, but why you would want to ruin a perfectly good bike by fitting one of those is beyond me