I'm guessing you fitted a Standard № LX-812, which is (one way or another) a Pertronix kit priced ridiculously high ($210 in America):
If it ran fine for 100 miles and then began running rough, odds look good (bad) for improper presence/value of a ballast resistor (see kit instructions) or a coil or distributor cap/rotor pushed over the edge by the hotter ignition system. Cap/rotor quality matters quite a lot; read this entire thread.
Other questions worth answering: what spark plugs are you running, did you know/remember the plugs' metal ring washers need to be removed before installing them, and what's the rest of the engine's state of tune, including its tappet clearance adjustment? Tune-up parts and technique suggestions in this thread. Carburettor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.