What type/size of MAG engine ?
Single camshaft, or more than one ?
Remember that setting the cam timing may have no connection with timing marks or setting the timing for the magneto.
The cylinder that will be firing will be the one where both valves remain closed as the piston comes up to TDC.
On v-twins, No 1 cylinder is usually the rearmost (behind) one.
On old engines like these, the inlet and exhaust cam timing is usually near symmetrical.
So if the cam timing marks are not giving you something like this, they maybe the parts are wrong for this engine, or marked wrong ?