Is it a flywheel magneto system, or does it require a battery to run? Maybe the flywheel system is for a generator, and the ignition circuit is really the battery (recharged by the generator), and a coil, timed by the points.
There are many different systems, but for an off-road bike of this age it might be reasonable to expect to find a flywheel magneto system with points? In that case is the points cam part of the flywheel? If so, timing is not too bad because there are two things that have to happen together: the magneto system has to be at its maximum flux point just as the points are opening, and the points have to open just at the right time - say 17-20 degrees before the piston reaches top dead centre.
I have to say that not having a key on a flywheel magneto is pretty strange. Usually the flywheel would be fixed to the crank shaft at a very specific angle (so that the the maximum flux point happens at said 17-20 degrees BTDC) then there would be some method of moving the points around to give fine adjustment of the timing.
If there is really no key way, then I'd guess the timing is set by moving the flywheel around on its taper. Or more likely it's just a generator, and the ignition circuit is battery point and coil.
Maybe photos would help.
Leon