Yes it is loosely based off of NKP #765, the preserved Berkshire, But you are correct, you can change the numbers to whatever you want!
To be honest to make the polar Express if you're using gold numbering on the locomotive already for pere Marquette #1225 the only difference would be on the tender, instead of pere Marquette it would say "Polar Express" then you'd just have to make the polar Express passenger cars. It would be cool though to have for the Christmas season. Plus this year is the movie's 20th anniversary