IIRC there were a few hardware "how to" schemes with in the old paper magz.
Polish Amiga Magazyn which was mostly - especially at the beginning - a reprint of the German Amiga Magazin had few of those.
I think there was something about "how to connect a 68010 to an A500" and "how to build an accelerator for the A500" or was it "how to accelerate the A500"? :/
IIRC You've couldn't use any 68000 processor, had to have a one with a specific serial number.
What I'm trying to say is, from what I remember, Your idea about replacing the 68000 - *can* be possible IMHO, with a bit of soldering here and there...
But don't quote me on that.