I use to breed rats, so I always had them on hand, and yes I used rats.
OK, in order of the least amount of trickery to most.
1 Put a rat pup in a mouse cage for a day, to pick up the scent.
2 Wash the rat pup with ivory soap wash well, do the mouse cage or rub on a mouse. (I know people that do this)
3 Do first part of #2, brain a mouse and run on rat.
OK, diff method
Feed a small mouse, enough so you can still feed more. As the snake finishes, hold a dead appropriate size rat and let him continue on the rat. This is really a power feeding trick. but do it once or twice (diff feedings), then as he is getting a good bit on 3rd feeding, wiggle the rat, although dead, he should constrict it then eat it. Seems to get it use to the meal. I use to do this on burms to get them off chickens.
If a real good feeder, let him eat a small mouse, if he is really ready for a second he may pop a rat just because he is so excited.
Honestly never did these with corns, but many other snakes years ago.
Good luck