I have assisted with a 12A conversion into a KP60, its' reasonably straight forward, but mostly you need to consider what the car is going to be used for. Most kits mount the motor quite far forward, which means that you won't need to mod firewall to get it to fit.
But having such a light car with a relatively heavy 12A sitting too far forward of the centre of gravity makes for terrible handling. If you're looking to do this properly, I would recommend sitting the motor back as far as possible, which necessitates modification of the firewall and transmission tunnel, but this allows for a significantly better balance and handling. Don't forget a decent rear end and brakes as well.
Or, if you're just wanting a KP with a rota bro, just fabricate/buy a chassis-mounting kit (which basically means that you've got 2 plates bolted to the chassis rails/radiator support crossmember, which the engine crossmember bolts to), make up a custom gearbox crossmember, get a custom driveshaft made up, find an SA22 engine loom to splice into the KP loom, install an electric fuel pump and lines, set up the hydraulic clutch, mount coils etc, you're away.
You may want to invest in a few spare KP diffs, (it doesn't matter wether you weld it up or not, you're still only going to get about 3 decent skids out of it!) and perhaps sort some bigger brakes off another early 80's Toyota (I can't remember what we used.. may have even been AE85?).
So yeah. Hope you either got mad hookups or deep pockets though aye, anything rotary these days is getting damn expensive and hard to find.. I personally wouldn't do it again, with setting it up properly for cert, you'd be better off just going out now and spending your hard earned money on a series 4 or 5.. possibly even a batty!
Just things to take into consideration.

Daily driver: Toyota RunX/Toyota Caldina
Ex: 2x AE101, 5x KP60, KP61, EP71, 3x KE70, KE72, AE70, AE82, 2x TE71, AE90, AE92, ST170, plus 11 Hondas, 12 Nissans, 6 Fords, 4 Mazdas, 3 Mitsis, an Isuzu and a Lada!
