The fiberglass bonnet would have been the first thing I would have listed in the description. This "oversight" may be due to ignorance however. My question would be, was the seller even aware that it was not original? Perhaps he too, was caught with his proverbial pants down.
Also, was there anything in the ad that indicated that the car was "completely" original? It could have matching numbers with a fiberglass bonnet and still be consistent with the description in the ad. Likewise, was there any language in the ad that opened the door for the possibility of aftermarket replacement parts anywhere?
I agree that you should price out the cost of having an original bonnet installed and go from there. You might also be able to sell the fiberglass one too and recoup some of the cost, keep that in mind.
All in all, I wouldn't let this "oversight" tarnish the excitement of your new car. It sounds like its a great car other than this unfortunate situation.
What does EBay have to say about all this?
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"
'68 Sprite 1275
'76 Midget 1500