Almost every engine Toyota made was avaliable in the *A60 Celica/Supra/Carina in Japan. So anything will fit if you can find the parts.
If you want something other than a 21R-C (a conversion), I would surgest either a 18R-GEU or 22R-E. The 18R-GEU is of the same age and fitting for that car, I'm sure lots of RA60's in Japan had a 18R-GEU. You will need a computer to run it of some sort, the factory one, or maybe some other factory computer like a 4A-GE one could run it.
The 22R-E was in local cars, RA65 Celica and the odd Corona, so all parts are avaliable.
If I had a RA60, I would definitely use a 3T-GTE. The best engine for that car in factory fitment/ performace terms. Similar cars were used in world rally.
Maybe a 1G-GTE would be better value for money. 3S-GTE is also posiable.