Aparently its quite common in Japan...
Have no idea if thats true or not...
I'm in the process of doing this to my KE...
It depends on how you want to do the caliper braket. Wheather you bolt it to the stock location or weld it to the housing.
If you wanna make it bolt on the useing s13 disc would be easiest.
But if you wanna weld the caliper bracket then use R31 disc as it has more bake spacing.
From my quike look at mine it apears that the caliper can only go in location because of the hand brake cable and its large bracket. At the front with the cable and bracket just under the spring perch.
I also noticed that s13 and factory AE86 rear disc brakes look uncanilly alike.