Rat pups are about the same as adult mice... Weaned rats are about the same as large breeder mice/retired breeders (extra large mice).

Go to a pet store and ask for a bag of the dirty, poopy substrate from inside their mice cage. It's free! LOL

Heating the rat up with some warm water (in a plastic bag, plastic bag in a bowl of warm water) also helps their feeding response.

Then dunk the defrosted rat into the bag and shake it and roll it all around, so the rat is very heavily scented with mouse smells.

Offer the rat and hope for the best. Mine both converted 100%.