There are more details about the emission standards, the factory equipment on your car and testing procedures for the 75 2.0L that you may already know about, or find interesting here:
http://bowlsby.net/914/Classic/Emissions.htm
Do you also have a functioning air pump/injection system installed? You should.
Post photos of your system to help us better understand which engine/exhaust you heve.
Assuming it truly is the original 2.0L (so many engines have been swapped) it sounds like the exhaust system may have been backdated to the earlier version. The factory exhaust for 1975 was part of the emissions system. It has heavy metal manifolds that help reduce emissions ('thermal reactors's), so if you are missing these that could be part of the problem. Here in CA, the seller is requried by law to provide a smog cert upon a sale, don't know Oregons' rules on that, you may have some recourse.
I have a complete factory exhaust correct for a 1975 1.8L/2.0L available if interested, everything from the heavy manifolds, the HEs, muffler hanger, 2 cats, a cat bypass tube and the factory 2-piece muffler, all in great condition. LMK if interested at jabowlsby-at-gmail.com