This page mentions how the checksum bytes are calculated...
The checksum routines in this game first add up all the even bytes in a memory bank and check against #00, and then repeat this for the odd numbered bytes in the memory bank. We do some quick math and discover that the new programís even bytes add up to #18B less checksum bytes than the program it replaces, and the new programís odd bytes add up to #187 more checksum bytes than the program it replaces. To fool the programís checksum we therefore add #8B to an unused even memory location and #79 to an unused odd memory location.
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