The engine from the S-15 will fit. You'll need the transmission and the rear end. To mount the transmission in your car you'll have to do a little reshaping to the tranmission tunnel. You need the rear end because of the speed sensor that's located in the rear. You need the S-15 ECU or an ECU that has a S-15 map loaded. And depending on the suspension mods, you might need a new drive shaft. Just make sure you have a S-15 drive shaft also.
The S-15 engine is the best base starting point, but it's also the most expensive starting point. The S-14 engine is cheaper and it's easy to get a T-28 turbo (new or replacement) to replace the stock turbo if the engine you get doesn't have a T-28 turbo installed. The 5-speed transmission from the S-14 doesn't require modifications in order for it to be installed.