You could put the drive mechanism from the external inside your Mac 128k. Or do the same to the 400k external advertised here (currently $10 + shipping):

http://cgi.ebay.com/3-Vintage-Apple-M0130-400k-floppy-drives-/120700862902?pt=LH_DefaultDomain_0&hash=item1c1a5501b6#ht_500wt_1043

The other benefit to that auction is, if you deyellow the cases and clean up the drive mechanisms to get them working again (even WD-40 would do it), you could sell them and get your money back plus a bit more for your deyellowing/cleaning expense, time and effort.