You may want to go to the , NHTSA and EPA web sites that will give you information as to what is required (problem will be that it is for current dates, not when you had your vehicle federalized). There will be phone numbers to contact individuals to obtain information. You may be able to get copies of your certification for a fee.
I am currently going through the process with a 1995 G 320. Depending on where you live and the yearly (if you have these requirements) testing that is done, it may not be possible or advisable to go back to pure European specs.