Check your points haven't closed up. Sometimes after a motor has sat for a while the cam on the dizzy gets a bit dry and wears the points quickly.
And check the points are good and clean.
Possibly change the condenser (fooled me a few times!).
And all the above stuff... clean your plugs, check you aren't 180* out, check and recheck the firing order (then check it again!).
Good luck, it won't be much of a problem.
Let us know....