just picked up a 86 sr5

[Mux213]

And drive the car around abit first. Make sure the rest of the car is in good nick. Also think about the fact that you don't want to just change the engine, suspension work is just as important as is the rear axle. All these are less appropriate to a higher performance car compared to a GTS, not to mention that after 20 odd years, suspension parts will have warn out anyways. You might even want to consider first improving the car, and take the engine upgrade for later.

That all said, look around in the project section. Plenty information around for swaps of different engines. Popular are:
- silvertop or blacktop (20v 4ag)
- 4agze (supercharged) or 4agte (turbo)
- nissan engine from a 200SX
- honda engine from a S2000

But all cost $$$$ to put in and require you to do your homework before attempting to do a swap yourself.