i would NOT...the stock gasket is a 3-piece gasket and is very durable. The new one feels very cheap in comparison.. I WOULD, however, get an new o-ring gasket for the midpipe. Trying to re-use a stock one is worthless when they are about $3 at your local autoparts store....I think the one I use is for a SBC (small block chevy)....hell just take the header with you so you can match it up in the store.
I wouldn't know, my OBX didn't come with a gasket (when did they start shipping those). You can try reusing the OEM gasket, but be prepared to take everything apart again if it leaks. I did reuse an OEM once for a short time, but I was also using the OEM manifold it was originally installed with...