That is a bummer. The guy who bought the kit was probably under the impression any 3 1/2 drive from a PC would work. That is a lot of soldering to make an interface cable for it.
Here is a 3 1/2 external Amiga Drive you probably could rip the 3 1/2 inch drive out off. Hope it works.
Ok all you retro-hackers. Here is another way to make money. I see a lot of 1581 kits floating around with no drive mechinisms. How about making a board/case that would adapted a PC Floppy (3 1/2 inch), which we know there are almost unlimited amount of at this time, to make it compatible with the 1581 kits. Reason I said case, is that the drives (not the case) look to be different dementions.
If someone made a case/mother board in which all I had to do is get a PC 3 1/2 inch floppy and it cost around ~$40 or so, I wouldn't mind buying a couple.