EPS is a very good option for our CNG fuelled (under-powered) vehicles, as HPS put extra load on engine.
In case of corolla you need to change steering rack (kanghi) from existing hydraulic one to ordinary one which can be powered with EPS motor. ( I am not sure if you can get one for corolla, I did this market survey for similar conversion for Baleno however finally did not go for it.)
EPS conversion in alto, cultus etc is cheaper & feasible since it involve only installation of EPS motor.