The surplus engine disposal shops should be able to get you an engine at a reasonable price.
As Scrubbie has already mentioned , any engine with the same shaft diameter & length will go in .
A Kohler should be plug & play while other brand will require a little work to fit the controls & wire the electrics.
Small Engine Warehouse
Small Engine Suppliers
Surplus Center
All good places to shop
Also note your engine is pre JS1995 so a new 20 to 22Hp engine will have the same output *** your current 25Hp because the way they are rated changed.
In reality you will not see any difference even from a 15 Hp engine unless you are mowing 1' tall grass & I assume the church gets mowed weekly, if not more often.