No, the MG wheels were never offered as an option on the 45, but as you have seen, some owners have fitted them subsequently. The centre caps for the MG Hairpins are the same size as for some of the Rover wheels, so the Rover centre caps can be easily fitted to them.
Not sure what you mean about the grille - both 45 and ZS had a mesh grille, the difference between them being that the Mk1 ZS had a shiny stainless steel mesh whereas the 45 had a black painted expanded metal one. The bumper mouldings are the same, so it is easy to retro fit the MG mesh to a Rover 45 bumper.
There was a version of the V6 engined 45 (the automatic I think), that had a seperate front number plate plinth which lifted the front number plate higher up (to allow better air flow through the grille for cooling) and this does alter the front end appearance slightly, making the lower grille more obvious. I know that several owners fitted this plinth to other versions of the 45 some years ago when the part was still available.