The problem isn't creating a disk with XP on it, it's creating a BOOTABLE copy. There's a small "boot sector" on the disk that needs copying, however if you just make a fully-copy image of the disk, you won't need to worry about it.
it installed but was missing critical files....
<snip>License numbers are not "paired" with a disk, only with the type. A Dell's number won't work with an OEM disk, etc. So long as it's a valid number, it should work fine.
This is not quite accurate. Microsoft changed the series keys with the produciton of XP w/SP3. You cannot use a serial number from XP w/SP2 on an XP w/SP3 CD and vice-versa.
I've made "slipstreamed" SP3 CDs that worked fine with my SP2 key, but they're an SP2 disk upgraded to SP3. Perhaps the same rules don't apply there.