Are the plugs fouling with oil, or soot? Soot is an over-rich condition, usually caused by the carburetor flooding. Oil fouling is caused by bad rings, or leaky valve guide seals. The guide seals can be replaced pretty easily, without a major teardown of the engine. Ring problems require an overhaul. There is a "booster gap" plug available for oil burners, that will resist oil fouling better than a standard plug. Mail-order places like J.C. Whitney sell them, or used to, anyhow.
Jerry
