Just changed the spark plugs. Here's a nice web page I used to get acquainted with the removal proceedure:

http://mbca.org/northtexas/spark_plugs_on_m104.htm

One thing I noticed is that the plug closest to the firewall (cyl #1?) had a fair amount of oil on it. The other five were prety clean. The area under the cover was oily in general. I cleaned it up a bit but wonder where the oil is coming from. For the plugs, I used the Champion copper plus ones with a 0.035 gap and 20 ft-lb torque. Ran the motor briefly and it sounds ok. Will take it for a test drive later.