Not difficult really. You probably have an ignition lead breaking down and the noise you are hearing is the leak of the high tension spark somewhere.
Get your mechanic to do a lead test with a high voltage tester and you will find it. Change the lead and all will be well.
The engine on the Super Cub is hinged by removing the cowling and bolts on the left side. The engine will then hinge to the right and getting at the mags to change the lead is no problem.
Have fun.