It doesn't sounds like the previous owner was the breeder which means it was most likely started on mice. At one year old, unless its under sized, it should be taking food larger than a weaned rat. Its recommended to feed 10-15% of the snakes weight in grams.

I would say just start with a good size mouse or rat without scenting with a chick. I'm sure it its eating chicks without issue a rodent will be no problem.

Did the previous owner say why it was fed chicks? I'm just curious.