If it's over the engine area, it's probably to make sure there's no fire (an inspection port) so I wouldn't be putting plastic or such in its place.
This vendor:
http://www.fisheriessupply.com/department/home.aspx
has about the thickest catalog I've ever seen. Not knowing the name of what you're trying to replace, I can't find it, but they might be able to help if you called them or sent them a photo or any information that's on the port.