Doesn't appear to be a checksum.
First test... I successfully changed it to "TRAINOER" by changing 0x151A2 from 18 to 19.
2nd test, I successfully fixed the spelling error with below changes:
The 'FE xx' value appears to be how it indents. So I changed it from 'FE 06' to 'FE 07' to push it 1 tile to the right, to center the text.
Then replaced the last few letters of "TRAINNER" with "TRAINER_"
Not sure what you did to get this game to crash, but it doesn't appear to be related to the text. Unless you REALLY screwed something up.
It just hit me.... what emulator are you running this game in?
If you're using NEStopia, it's possible NEStopia is relying on the CRC to figure out which hardware to emulate.. so any change might make it fall back to using the iNES header.
But the header should be fine... mapper 9 doesn't really leave a lot of wiggle-room... Punch Out (all versions) is the only game that used it.
Or did you... cut the file size? Like actually delete a byte and shift everything after it over by one? Because yeah that will bork it bigtime.