You say you can replace OR repair. I'd suggest replace. The repair tape is a bit thinner and if a portion of the wheelwell is damaged, the remaining part of the DARCO is very likely to be damaged and/or punctured by small rocks/stones that have been slung into it through the years.
You can buy replacement DARCO at any RV parts center, mobile home supply store (it's the same fabric that's used to cover the bottom of manufactured homes). The tape that's used to repair it is called SCRIM tape and is available in 2,4,6 inch and wider rolls. Keep in mind that it is a "film" and has a paper backing that needs to be removed to apply the tape. I find that anything wider than 6" is extremely difficult to apply properly because of the problems handling the tape while trying to remove the paper backing. I prefer the 4" SCRIM tape for most applications.
If I were you, I'd inspect the undamaged wheelwell carefully. Chances are there is water intrusion (through those small pinholes) and likely water damage to the underside of the wood used to form the wheelwell. I recently installed 1/4" HPDE sheeting to line my wheelwells. Although quite a bit heavier than the DARCO, it is structurally much stronger and I believe it will protect against those "pinholes" much better than the DARCO. I did leave the DARCO in place (and suggest you replace yours) as a second line of defense against water intrusion, under the HDPE sheeting.
There are some photos and a discussion of what I did in the thread located here:
http://www.keystoneforums.com/forums...ad.php?t=39472