I attempted the same edit to the cracked Ms. Pac-Man many years ago.
There is a checksum performed on the title text which will cause a
crash if altered.
As I recall, there is a jmp (or jsr) at the start of the file to the
checksum routine at the end of the file. Bypassing the checksum
routine simply involved NOP'ing (EA EA EA) the initial jump. Then the
title text could be altered safely.
I still have the patch written down and archived away with my notes if
you're interested. Although it shouldn't be too difficult to find it
with the info I just gave.