Would it be possible to replace my '94 2.0L 8V with an earlier 2.0L 16V?

Possible, YES. Worth it? Not in my opinion.
Unless you have a 16v for $0 and your 8v has a hole in it, why bother. I have had a few 8v vw's and they may be slower stock, but they can at least get to the 136hp of a 16v with some bolt ons.
I had a quick jetta with the 8v motor with only minor mods.. then I went a bit father and it was very quick. Personally... I have a 16v passat now and wish it had the 8v engine. maybe I'm not the norm but I say Build what the good lord gave ya:)