[i]Is there just one size of Heat Exchangers which fit all size engines; 1.7, 1.8, and 2.0L's?

Any of the 914 exchangers fit any of the 914 motors. You will need the appropriate muffler, and muffler hanger. The 75-76 exhausts have different heater hardware, as well. But you can swap everything if you want to.

--DD