Are you sure it is tightened down and the head bolts are tight? I ask just because I have not run into a leaking plug before but did have a head bolt make one look like the plug was the culprit.
More asking here, but if it is the plug, anti seize? Is like a thread sealer but is to keep threads from bonding. They make special stuff for plugs, but I have never tried it. Some one will know and better advise you. I googled it and came up with some interesting reads. spark plug thread anti seize - Google Search
As a way of help seal them)
Great to hear your enjoying your motorized bicycle so much. These things are great.