Swapping classic mini engine?

How easy is this to do. Theres a car on ebay that i love the look of, but would want to convert it back to a 1000cc engine, as opposed to the 1.3 cooper engine that the seller has had put in. My dad is good at mechanics, but how long would it take? Does every part under the bonnet have to come out? Is it just part of an engine that makes it a 1.3, like a block that you just swap or something? I apologise for my ignorance Thank you for any help and/or advice!

It's fairly trivial, but you might want to consult a specialist about what is the correct engine for that particular model. The value is less if they are not original.

OLDSMOBILE 455 W-43 EXPERIMENTAL HEMI AND OTHER EXOTIC ENGINES - THE JOHN BELTZ YEARS PHOTO TRIBUTE