Does it need to be a GNX engine? A normal GN and GNX have the same engine internals.

Doesn't really make sense to pay for the GNX motor (assuming you can even find it) when you're probably going to get rid of most of the stuff that makes it different from the normal GN.

Here's the differences: http://home.att.net/~buickGNX/GNX/engine.htm