I would try and get a 1994 c20xe with DIS from a Calibra. Not sure how much room you'd have in the Gemini for a distributor near the firewall so better to start with an engine that doesn't need one.
A 2.2 or 2.4 (c22ne/c24se/y24se) rodeo/Colorado gearbox should bolt to the c20xe, you may need to use some other flywheels depending on how the input shaft lines up with the clutch disc. The 2.2/2.4 sumps 'may' work in the Gemini they might not so expect the worst and budget for custom or even dry sump set-up. You should find that all the engine mounts are the same.
If you got a Calibra wreck I wouldn't be surprised if you could use the engine management also. But I guess in a build like this you might as well go custom their too then you just gotta decide if it is injected or carbied.
Hth
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