Look at the P89C664 part from Philips. If your board had a standard type PLCC packaged processor, you should be able to drop this part into the same socket and immediately have 64K of FLASH. The P89C51RD2 is 64k od flash and available in DIL.

I have had to upgrade software in controls installed in animal confinement facilities (fancy way of saying 'Hog barns') The control boards have a pair of 29F010 chips (1 for program, 1 for settings and historical data) on a 'memory card' that is plugged into a header. The upgrade involves opening a case and swapping the card. I hope that I NEVER have to bring my notebook PC into that environment.
Why not, in that case put the uC onj the 'memory card'

Erik