Posting this as a reference for anyone who may need this in the future. The NGK XB05F (8062) spark plug cap ($7 on ebay arrived in 2 days) is a suitable replacement for the Moto Guzzi OEM part (1A010839 $45 who knows how long). I was able to simply twist the replacement cap onto the original spark plug/ignition wire. The spark plug is very recessed on these bikes, requiring a long reach on the spark plug cap.
I broke the original cap when pulling it off the spark plug to do the first valve adjustment. The NGK cap requires a threaded end on the spark plug, so you need a different plug (NGK CR8EIX $10 on sale at West Marine of all places). The original cap was on there so tight that I actually bent the tip of the spark plug. Lesson learned- twist the cap before pulling...
