... are there any good mods out there that keep the mechanics the same but have different ship models for the various units?
Well, why not make your own mod... you always can download other mod, rip the ship that you like and create your own mod...
If you keep your own mod private, it is not a problem... if you make it public, be sure to have the right credit and permission for use...
Since you wish keep the basic game mechanic, it will simply be a work of rename some mesh file and a little edit in entity file... not really a big or complex work...