I can tell you all our PS-2 early engine with the 8.4 battery are still running just fine. Of course we've changed the battery and on some we're using BRCs. Most of the early engines were designed to run on 3 rail track only and therefore have no issues with out side rail pick up. Anyway I have a 5 volt ps-2 from around 2000 that still operates just fine.
Apparently the speaker is the first that goes wrong with these early engine. We haven't changed a speaker yet but have ordered a couple just in case. One engine is getting a little scratchy and weak.
Maybe we've been lucky. I like the early engines.