Back in 2012, I was freelanceing for Hornby Magazine and in my "Parkers Guide" column, pushed the envelope a little with some simple scratchbuilding.
Taking the Dapol (now Hornby) L&Y "Pug" as the basis for a model, I scraped the entire body away and built a new version using Plastikard and a whitemetal chimney. The end result ran as well as a Pug does, which is OK, and looked (IMHO) pretty good.
Best of all, I know the article inspired a couple of people to do the same project, and both were as happy with their locos as I was with mine.
I suppose nowadays, there would be the cry "can you 3D print it to save me some effort" but that defeats the object of the exercise. Nothing here is difficult, you just need to take your time and throw away any bits you cut out that aren't as good as you would like.