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 ass 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.