That is probably this easiest way to do want you're wanting in this case. I agree the hole through the engine really is a bad deal, but it is an easy fix. What I have done in the past was to cut pieces of Plastruct to the shape of the opening and glue in place. A small amount of body filler closes any remaining gaps and a little sanding and you never know it was there. I have used this on several vintage AMT kits also.