Personally, I've not had much luck with epoxy and covering.

Epoxy often developes a waxy surface that covering won't stick to, and epoxy doesn't stick well to plastic. So having epoxy either under or over the covering isn't a good idea.

You said this plane is used - That's the problem. Once the engine compartment has seen fuel oil it will be tough to clean it all off.

If it were me, I'd clean it good with alcohol or acetone, and I'd be prepared to repaint if necessary (Acetone might eat the paint). Once the surface is CLEAN and fuelproof, apply the covering, then wick some thin CA along the edges of the covering.