I Nathan welcome to the forum. I am no Enya expert but have owned and operated a few various ones in the past both controlline and RC versions.
Now I would be willing if it is a .35 ci displacement engine to wager a Stale Glazed Donut aginst three buned out Glow Plugs that the spark plug you are talking about is actually a glow plug. A picture would be of immense help
Your link to some E Bay sale above does not work.
if it is a .35 Cubic inch engine then that seems a very likely possibility as I believe there have been Enya's in that size both for controline and RC. You can tell the difference easily the carburetor just forward of the cylinder is just a big hole with a needle adjustment running through the side of it. The RC version would have an articulated lever out the side of that hole (venturi) that rotated a barrel in the ventrui.
John