Why bother w/ electronic? At the RPM most flatheads turn, the stock ignition (w/ a 12V coil and heavier condenser, if using 12V) will work fine.

If you don't have the stock parts, you still can buy replacements fairly cheaply. If you still want electronic, Mallory makes a breakerless distributor that bolts on (MAL-3727501), but it costs over $300. A single-channel MSD Stacker system (P/N 7200) w/ a Blaster Coil will provide a good spark, but not before you spend about $150.

Anyway, good luck.