Generally, the one component you might replace is the "zinc"- the
protective anode rod; likely way beyond time for that.

I'd prepare to replace heater in the next year or so- monitor it for
any leaks or odd behavior. Pretty simple process, especially if you
have all the fittings an tools you might need ready. Newer units have
additional safety devices, and are capable of higher efficiency than
yours.

You NEVER want a careless person twiddling around with something like
that t-stat; sorry, but your wife qualifies. At the very least, someone
could have gotten very badly burned- boy, are you lucky!

What I find much simpler is to leave the t-stat alone, and turn the
control from "pilot-only" to "on" or vice-versa. But then, mine is not
in garage, so there's no threat of freezing.

J