There is not a real problem running NTSC games on PAL Amigas (the minor drawback is the game will play a bit slowly, but I call it a good feature, since you can beat the machine more easily :-D).
The NTSC screens are 320x200, PAL is 320x256, so any NTSC will play, maybe with a black border under it. The other situation is the problem: you can't "fit" a 256 lines screen in a 200 lines resolution, so this is why PAL games don't work on NTSC machines.