Identifying engine without paying

Hoping someone can help me with this question on my 1988 Subaru DL wagon. Been frustrated working on the engine to be getting all the wrong parts, and finally came to the conclusion that someone must have swapped the engine somewhere along the line. What I am trying to find out is how can I determine which engine I now have? It's an EA-82, fine, but I'm trying to find out what it came from, if possible, how old it is, how many miles it could have on it, etc, but above all I WANT TO BE ABLE TO GET THE RIGHT PARTS THE FIRST TIME. Any advice would be appreciated. Matthew