The short answer is: it's a lot of work.
You might have heard of GTA IV: San Andreas
. It's doing essentially what you're asking about but with the GTA: San Andreas map instead of London. Take a look at the team credits and you'll see there's quite a few people working on it and it started in 2011. They're presumably not working on it full time, but still you'll get an idea of how much effort is involved in something like this.
Using The Getaway like you mention might be possible as the SA project has taken a non-RAGE world and imported it to RAGE, but I really don't know enough about the nitty gritty myself.