I have seen 1 S series LSD advertised in Australia in the last 5 years. T series requirements have been well documented here, toymods and AE86 Driving Club. Wherever T series diffs were not fitted in AE86's. i.e. Australia. It's a bolt in conversion but expensive. T18 diffs T series diffs are bolt in but hard to find, and pricey when you do, plus drum brake. AE86 drum T series virtually unseen here. AE86 disc rear T series available but very expensive.
Read up on T18's diffs and sway bar fitment. T series diffs and axle sizes and diff centre compatibility. It is very well covered everywhere. T series came out in many other Toyota's but only the T18 or AE86 housing is a direct bolt in.
The search button is your friend.
But yes, be careful about rust, and wary if it has been modified, unless you know what you are looking at.