[Cobrajet67]

Hazey is spot on here. Just to add that if you buy a complete engine from anything other than a compatible year Mustang then you'll need to swap the original Mustang intake manifold and exhaust manifolds over to the donor engine and other things as well like the fuel rails, oil pan, and pretty much anything else that's even minorly different.
A complete 4.6 out of any 1999-04 Mustang will be hard to find right now. They used to be plentiful on Ebay and in the national salvage yard database(none at all on the LKQ site) but they're very hard to find now so you'll likely be better off getting one from a low miles 01+ Crown Vic and just using the long block from it and swapping everything else from the old engine over. Here's the only complete Mustang specific one on Ebay and it's not cheap: 99 00 Ford Mustang GT 4.6L SOHC Engine Motor Assembly 100K Mile Take Off X33 | eBay

 

[Cobrajet67]

That's disappointing to hear. If my 02 engine goes bad I guess I'll have to buy a new car or go forward with installing a Coyote.

Complete used engines are just for those people that want the least amount of work to get the car back on the road. It's not usually the best choice since you don't really know what the actual lifespan/condition of the engine you're getting when it's used. As Hazey pointed out there are other vehicles that used the same basic block and heads as the Mustang did and are perfect replacements such as a lower mileage Crown Vic of certain years. You simply need to switch over all the Mustang specific stuff to the donor block.
The best choice really is to either buy a remanufactured block or just have yours rebuilt or remanufactured. A Coyote swap would definitely not be a practical, simple, or cheap solution.