Clinton,
The first thing I would check is that you are using the correct grade of spark plug - NGK B7HS seems to be the preferred plug these days. From the symptoms you give, it seems that one plug started to foul up, giving the other cylinder more work to do, thus making it run hotter, which in turn prevented that plug from fouling. If you are already running B7HS plugs, it may be a good idea to try B8HS plugs until you have run the motor in and are prepared to make it work a little harder.
Also, Lucas magnetos are not provided with a 'safety gap', so it is not a good idea to make the spark jump excessive gaps as it can cause the spark to jump across the windings, causing more damage.
Regards,
Eddie.