Hey, gang. New to this particular forum, not to the Fiesta. scotman might recognize my name from another Fiesta forum. I may be able to contribute, as i'm in the middle of doing just this upgrade at the moment.
What I'm doing is assembling a complete strut assembly and doing a direct swap, rather than disassemble the originals. I've been mostly opting for OEM parts, except those that I've found were on indeterminate back orders.
I have the KYB rear boot and bumper, and it does look like the stock unit from what I can tell, based on the illustrations at websites like Tasca. It also looks like the Moog part that Lions Tooth posted from Rock. I think the key is they list it as, "OE," style, which should be equivalent to the dealer part.
I'd recommend the OEM dust boot for a couple of reasons. If you look at the clip at the top of the strut body, the spacing between the hooks is sized to accommodate the width of the bottom-most ring on the boot. You can work the boot into the clips to keep the bottom of the boot in place and not bounce around or lift upwards.
Also, the top flange of the boot appears to be intended to act as a spacer between the spring and the seating area on the underside of the bearing. Or maybe it's just supposed to be held in place so that it doesn't wander off. I'm not sure. The factory manual seems to show the flange as placed between the spring and bearing.
If you guys have any questions or need any other photos, let me know, I'm glad to share this info.