The problem with a disassembler is that it does not know the difference between code and data.
Find a disassembler that allow you to modify the input (or recalculate the checksum of the .hex file). The disassembler will go awry every place which is not actual code but fixed data such as constants and jump tables.
1) disassemble the kit and kaboodle, save that file and about once an hour going through 2), 3) and 4) save it again, I will refer to it as X.
2) Locate text strings (let your editor find hex 20 and evaluate each occurance). Replace the disassembled gibberish in X with the appropriate db 'abc..." strings and replace them in the input file with "NOP"s
3)The start and interrupt vectors will point to actual routines, if, at those addresses there is gibberish make a note in your log of the contents of the 3 previous locations, replace them with NOP, that will resynchronize the disassembler.
4)Periodically, after much blood, sweat and tears, assemble X and compare it to your original input file, trick: load it into your programmer/ICE or whatever you used to get the file to disassemble to get the same layout of the Hex file. If the compare fails, go back one 'release' of X and start again.

when things look reasonable, insert the dropped data from your log into X.


Is'nt this fun?

Erik