engine rebuild

If you're lucky enough to have a RELIABLE one man shop in your area you might get it done a bit cheaper than a commercial exchange engine but be sure you're getting the same new parts installed and the same level of work (cleaning, machining, etc) performed.

And Stumpalump's recent report here of a rebuild gone South is an example of the potential for problems. Most commercially remanufactured engines come with at least a minimum written warranty. I've only had to avail myself of that warranty once and that particular company provided a replacement motor and several hundred dollars cash to cover MY labor for the R&R of the unit.

YMMV,

TOH

With an "N", almost certainly "oil" in the radiator is NOT actually leaked crankcase oil, since there's no way for that to happen, except POSSIBLY an (uncommon) oil gallery-to-water jacket crack, but rather, combustion products that have leaked over a long time past the head gasket, and accumulated in the cooling system.

So, it is quite possible the "oil" in the radiator is totally unrelated to the engine stalling, and the noise you heard. You really need to get a good diagnosis of why it quit, before being certain an overhaul is needed.

A film of "oil" in the radiator is certainly NOT desirable, however, having coolant in the crankcase is 1000% WORSE, and will eventually cause the bearings to fail. A SERIOUS head gasket leak will allow water (coolant) to get into the crankcase oil, as well as "combustion products" to get into the radiator.

Have you loosened the engine oil drain plug a little, to see what comes out... any coolant that has leaked into the crankcase will settle to the bottom, and drain out FIRST, before the oil.