The inside of the Lisa is cavernous.  If you had a 12" monochrome Mac Monitor('s guts attached to the Lisa's tube or replacing the Lisa's CRT entirely), you could pretty much drop any monochrome capable Mac LoBo in there.

I'd do it with a SE's guts, so it would just be a nice, simple, slightly-overclocked Mac XL with an internal SCSI HDD, and square pixels.