On a 91, it's tougher to do a V6 than an upgraded 4cyl. It's been a while since I visited this area of thought, but.
The 1.6L and 1.8L used the same head. If the 2.0 used the same head, THEN you can just swap in the engine Cover to Cover, leaving the exhaust and intake from the 1.6L in there, bolt them on and your good to go. You won't get ALL the power of the 2.0 as it used Multi injetors, however you'll have a Throttle body 2.0... Someone correct me if I'm wrong here PLEASE.
OR and most likely easier even...
Get a head gasket from your local parts store for both the 1.6L and 2.0L. Make sure there identical, I'm thinking the 2.0 used a larger bore so probably not. If they are, then just swap over the block... However something tells me the 2.0L was DOHC and your 1.6L is SOHC so this may in fact be harder...
There there is Trail Tough's conversion... Basically you'll splice in your single injectors wiring to the 2.0L's 4 injectors (one pulse makes all 4 open-this is normal for non-sequential fuel injection). Everything else from your 1.6L wiring wise will need to be used including the Air-Flow meter and o2 hookups...
Still without actually comparing them side by side (best way to do this BTW) It's tough to tell what to keep, what to not, and what to splice and tap.
All things considered, if it were me I'd get the entire 2.0L drivetrain including the t-case and swap all that in. Nah, I'd just buy a 2.0L trackicker and be done.
V6? a 90* V6 like a Vortec will need a large shoehorn. It's possible but it will be tighter than you want it to be and require custom headers. The 60* V6's, 2.8L/3.1L/3.4L would be an easier fit however mating to the tranny wouldn't be easy. If your running an Auto then you already have a GM tranny, TH180 I beleive.
Just keep in mind the more power you send down the pipe the more things needed upgraded down the pipe. I ran with a V6 (4.3L) sami once, it broke the intermediate shaft 20mins into the trail... And we were not on anything hard yet.
The 1.6L is lonely with it's 90hp. But gearing makes a world of difference in those squirrels than power does. Cat back, snorkel, headers, and gear it right.