I remember reading a very entertaining story as a kid by John Christopher (typically an author of juvenile sf) in which basically the opposite happened (the protagonists are transported to an alternative 1981 in which the Roman Empire survived in stable form basically unchanged, and they're able to transform its fortunes with longbows and pendulums... although helping the underground Pope legitimise Christianity as a religion doesn't go quite as well as expected. Special props to the author for having the imagination to conceive that the pendulum could be used as a torture instrument!
