For Ninja Garden Black (the best version of the first 3d one) it's still either original hardware or backwards compatibility on an Xbox One. You can emulate it through Xemu, but I've always found Xemu a little rough performance wise but that could just be my hardware. For Ninja Gaiden 2, it's still locked behind original hardware only as Xenia (360 emulator) is still really rough last I checked.
You could always get the latest Master Collection release which has Ninja Gaiden 1-3, but they are the 'Sigma' re-release versions of the first two. Sigma 1 isn't bad and is more or less just a few new weapons and some item changes, but Sigma 2 is a drastically worse version that I do not recommend at all.