Which speaker? Hardware advice

Hi all,

Im trying to update a retro radio setup i did a little while ago… because i know it could be better

This is my raspberry pi set up at the moment and the first thing i need some advice on are the following questions.

  1. Should i upgrade the pi to a 5? (Or 4)
  2. Should i change the amp zero to a different hat?

Hopefully the next pic will help with the next questions

This is the space i am working with, rough estimation of free space is. L 4" W 8" D 2"

It leads me to my final question, this was the old speaker and i am useless at this bit

  1. Can you recommend a 5 inch mono speaker that would work with the existing (or recommended improvements from initial questions, both preferably) hardware.

I would be delighted to add a new 5inch speaker and get much improved sound quality.

Thanks so much for reading this, appreciate any help