You have to cut a groove in the head and set a stainless wire in with butt ends, like was stated above. I used to do this all the time for V-8 Blower applications. We used an .051 wire in a .049 groove, set about .035 deep if I remember correctly. We still used thick, solid copper gaskets (.100) in combination with the wire, but it wouldn't be needed for a moped.
A std head gasket with matched surfaces should be fine unless you got some crazy ass comp ratio and/or over/under torque the head. Use a good, clip-stop torque wrench and you shold be fine
You don't want to use rubber.