nope, the Supermax and Navigator carts aren't compatible, sadly.. iirc the Navigator uses standard OneBus but the Supermax swaps some of the registers around or something.
and the original Power Joy uses a 9v centre-negative DC power supply - you're right that the one from the NES won't work (as it outputs AC). if you have one from a model 1 Mega Drive/Genesis that should work, as will the original Famicom's or the majority of other Famiclones. Otherwise just get a universal AC adapter.