I believe macmetex still has the 33MHz modules for a buck for two, and the 25MHz 520 modules as well. If you really wanted to go ahead with this, perhaps you could buy up enough of his to match the number of full '040s purchased, find someone to do the rework, overclock & test them, and then dribble them out slowly back onto ebay. If you can get the rework done cheaply enough, * there should be a tidy profit in it.

* or, indeed, if someone here could do it. Heck, let me get some practice at toaster oven reflow in first and I might think about doing it myself.