1.6L bottom ends are the same - the difference is in the head / intake / fuel injection & ECU. So, yes, you can toss in a 16v bottom end, and your 8v heads. Samurai guys do this all the time (the 1.3L head will bolt up too). The 8v head is the same in the 1.3L and the 1.6L. The valve seats may be okay, it's far more common for the valve guide seals to go out (read: cheaper & easier).
8v = 80 HP / Tq
16v = 95 HP / tq
2.0L = 130 Hp / Tq
If I were doing a complete swap, I'd be looking for a 2.0L. A lot more power for about the same $$$ (if buying commercially). Check out aftermarket4x4.com - Sean's got several writeups on engine swaps on there. That said - I've seen complete 1.6L go for pretty cheap (private party) in my area ($300-$600 NorCal). Geo Tracker IS the Suzuki Sidekick. Geo never manufactured a single vehicle - they only re-badged other vehicles (Suzuki, Isuzu & Toyota). Also keep an eye out for JDM engines. If frankensteining engines doesn't scare you, in theory you may be able to use heads from Geo Metros (Suzuki Swifts). Check out teamswift.net for more info.