This upload of mine probably counts as both building a level with a found tileset and modification of one. The tileset used there is a slightly modified version of IceDileria which was originally uploaded here and you can notice it was itself an extension of earlier tilesets by the same author: Minimaze and Minimaze MK2. I didn't remake the tileset and only extended it by useful tiles that make good usage easier, plus tiles I needed for the script used in the level.

Also, while I'm on topic of Thrifty and already mentioned three of his tilesets, it would make sense to link to the only remaining one, which also has a clearly misspelled title but otherwise looks functional and not too bad.