Curse you for not putting this in the balsa section. Didn't see it until now!
It looks like whoever built this initially did a good job at least. I wonder who made this kit? Surprisingly few manufacturers have kitted Ercoupes, a real shame if you ask me because I think it's quite a pretty ship.
I would have personally side mounted the engine, it would look sleeker without it exposed out the top. Most 4-cycles will run perfectly fine on their sides. But, now that the hole is cut there's no going back really. At least in this configuration, there will not be any cooling problems.