There have been so many changes of infrastructure ownership since Keswick closed that I'd expect it would only be by chance if any drawings have survived.
The only possibilities I can think of are local authority planning records if there were changes made to the building in the BR era, or the archives at the NRM, though I don't know whether they can search on your behalf or if you have to help the search yourself. However you may well have thought of those already.
If you only need the site layout rather than an internal plan, the National Library of Scotland maps site has 25" to the mile maps, though there are restrictions on commercial use of these:
https://maps.nls.uk/view/121145246