Oil on the sparkplug can mean the oil rings on the pistons are gone. Happened to my Mini, which led to a total engin rebuild. If the garage guy says its the rocker cover gasket... well thats odd. is the oil on the spark side of the screw or the ceramic part?
Replacing the rocker cover gasket is easy. Take the cover (might require some tugging) + pipes off (remember where they all went and write down how many thing you take off and where they go!) Then gently remove the rubber rocker cover gasket.
Might wanna see if it really is broken at this point and see if there are any sharp edges when the old one made contact with the cover and head.
Now give the whole thing a good rub with some oil on your finger (the stuff on the head will do) and put it back on gently and make sure its all in the right spots. Then gently put the cover over the top and MAKE SURE (!!!) its all sitting nice. No over lapping rubber sticking out or disappearing in. Then tighenten the cover back up but not too hard. This doesnt need to be bolted down with the force of 20 elephants. A quick pinch and itll be ok. Replace all the pipes and give it a start...
Dont take it for a 200km journey yet, run around the block, check the gasket, check the oil, check the plugs... then repeat. If it still doesnt solve the issue, get another mechanic to check it out.
