The difficulty is, as usual, finding out which one to change. ;-)
Later I used the +fravia site for hacking protection systems in much the same fashion - disassemble, convert the checks to NOPs, and patch the binary.
I've recently tried to go back into reversing, but one problem is that there are very few Linux binaries which even prompt for serial numbers!
http://crackmes.de/archive/
The difficulty is, as usual, finding out which one to change. ;-)