Looking for original Cobra R, or just "Ford Motorcraft" in general?
If you're looking for a genuine original 2000 R serpentine belt, try reaching out to Jim D @ PPI, he has the originals - they are very hard to find and not cheap. He is known to have rare hard to find Cobra and Mustang NOS parts:
If you're looking for an off-the-shelf, don't care if it isn't NOS, then according to the specs, this off-the-shelf DAYCO belt may be the factory replacement:
Find many great new & used options and get the best deals for Dayco 5060830 Serpentine Belt for YR3V-8620-BA Q4080830 K060832RPM K060832 ij at the best online prices at eBay! Free shipping for many products!
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I would reach out directly to DAYCO to confirm 100% of the cross reference for the above belt.
GATES drive belts is another source too for serpentine belts; again you could reach out to them directly for a cross reference of compatibility.
Both Gates and DAYCO are high quality off-the-shelf serpentine belts - I've used both in the past for daily drivers and high performance applications.
And there was this older thread from a few years ago with some info (and another reference to the above DAYCO replacement):
One NOS Ford FEAD serpentine belt for the 2000 Mustang SVT Cobra R model’s (only) 5.4 engine. This YR3V-BA belt is unique to the 2000 Cobra R and its production at DAP. Impossible to find now and this is our last one.