To properly mount a new valve, you will need access inside the wall, either from the front or back side.
Depending on the layout of pipes and studs, it is possible to preassemble some of this, but you still really need to be inside the wall. A good plumber can sometimes replace a valve just working through the hole in the tile or fibreglass. It is very difficult, not a DIY project.
If you use PEX, there is a whole new field of knowledge you need to study up on....fittings, tools. And, remember this: PEX sizes differently than copper, and if you use PEX to install the tub spout, you will be posting back here to ask why water comes out the shower head when taking a bath! We get that question about once a month on a regular basis!