I want to put a Life-Like FA1 shell on a Kato FA1 mechanism. Has anyone ever done this? Is there anything I should be aware of when trying this?
I am going to guess that what you mean here is that you want to put a Life Like FA1 shell on a brand new ATLAS (not Kato) mechanism. Atlas bought the Life Like tooling from Walthers (who had purchased it from Life-Like), and the current Atlas FA1 is a retooled version of the Life Like. My understanding is that the shells are easy to swap, and I believe some of the denizens of the Railwire have accomplished it- so you may want to do a Railwire search function.
Do beware, if you are purchasing the makings for this on eBay or other second hand sources, there was an earlier FA1 from Atlas, made by Roco maybe 45 years ago. It was one of my first N scale diesels. Was a very fine loco for its time. I mention this just to alert you that not everything sold as an "Atlas FA1" is their current production, so be sure you know which set of innards you are getting.