From ThomasW's comment linking to Wikipedia, I've learned that the original AirPort card was "a re-branded Lucent WaveLAN/Orinoco Gold PC card, in a modified housing that lacked the integrated antenna."

While it theoretically might be possible to take a Wireless 802.11n PCMCIA card, remove the housing, and retrofit the card into the proprietary housing used by the  PowerPC G3, I wouldn't recommend it.  The OS would not support any other brand of cards on the hardware level, and the download speed would also be capped by the Mac OS Classic software settings.