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I don't think there is an after market airbox. I've not seen one anyway. CXman may know or ask that at NGWclub maybe. I do have a K&N filter in mine that came in my parts bike though.
Thats a good ideaMost swear by the Honda head gasket. Search carefully and you can find them at a reasonable price. I've never seen an aftermarket airbox. Probably because it is hidden in the shelter. BTW, before you put it back on, put something like screen wire over the snout to keep mice from invading in the winter time. They love to nest in them.