Been almost there and nearly did that--a 2.0l 1979 Bus engine into a 911.
Not nearly as difficult as detractors would make it sound. (But not
somehting I'm in a hurry to do again--it moves much better with a 911
engine.) A WBX with a 228mm flywheel MAY bolt right into the 901
transmission that 912 used, but not into the 915 middle/later 911s have.
Hook up a fuel line, ignition power, instruments, and then ALL you have to
do is worry about plumbing. The 911/912 doesn't have the room around the
transmission that we v-dubers are used to, but maybe a smaller radiator in
the decklid like used for the air conditioner, or small rads in all the
wheel wells....
Compared to the ridiculous prices for just about any Porsche parts, WBXs are
budget motors. And reliable, maybe even quiet (mine never is).