I'd clean and inspect the plugs. It could be a bit of lead/carbon/oil fouling one, but it could also be a duff plug.
You can try swapping the top and bottom plugs on one side.
If it changes to rough running on the other mag then it is probably one of the bottom plugs that you moved to the top.
If it is still the same mag that is giving the problem then try doing it again on the other side of the engine.
If after this it is still the same mag then it may be a mag problem.
(this is because each mag will probably run the bottom plugs on one side and the top ones on the other)
Changing a plug is probably cheaper than calling an engineer out. Most engineers would just clean and change the plugs first so you may as well do this yourself.