If you know where to buy a proper Motobecane stroke limiter, I'd like to know! sadly, while this would be a useful tool, it's not exactly the easiest thing to find.
Sorry, we make do with what we have. I use a hammer over a pipe becuase the increased leverage of the pipe is more smooth but it's easier to possiby damage the piston or camshaft that way. The hammer hits need not be huge, mere ly enough to loosen the cover plate. If the magneto cover is jammed on far too tight, other things like penetrating oil, heat or WD40 should be tried.
I learned the Inertia Trick from a career engineer who used to work on mopeds when he was younger, so I think I'll stick with it.