On a well, add a valve near that.
For this, many redneck options.
Given it is difficult to get to, start with simple.
How old is house?
You need to know this, what supply houses were around during that time.
Turn off well, drain house so you can remove the rest of the parts.
Drive to said supply houses with parts in hand and photos of the rest.
You may fine a new same unit that you can use for parts.
Other option.
With lathe, drill center of both sides leaving enough meat to be strong.
Drill to fit a rod that can be used to slice together.
Cross pinning optional.
Solder or good epoxy to bond it.
File to shape and replace.
It should not need but small amount of torque to seat, either it was weak or over tightened.
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