I inherited a Mira 88 when I moved into my current home
I understand that they should be survived every year, and the gasket set replaced. This one clearly had not been touched since Noah landed
All the plumbing was hidden within a stud wall
Local plumber removed the old shower and replaced with a thermostatic power shower for £120 ( shower, two new shut off valves, new electrics and labour)
depending on your financial state, and the ability of your hubby, it may be easier to get a plumber to do it. remember the (power) shower will cost you £70 / £80 anyway
Regards David