C,
Check this out - FSM has a review of an Academy P-51B. I don;t know if it's the same issue - but it says that it comes with an optional closed or open canopy. Plus it'll give you other info about the kit.
http://www.finescale.com/fsm/default.aspx?c=a&id=137
I built Academy's C version. Compared to the Hasegawa D and a Revell of Germany B that I've built, I thought it was the best of the bunch. Very nice detail all over the surface, in the wheel wells and in the cockpit.
I might have overdone it a bit on the weathering - it was an experiment.
Chris