I would take an area of all 00's, and change one value to 01 and see how PKHex recalculates checksum. Then change it back to 0, and change the next offset to 1 and see how that changes the checksum. Then change the value back to 0, and change another 00 to 0x2 or 0xF, and see how that changes the checksum when PKHex recalculates it. You will find it is arithmetic and you will be able to easily find a pattern of changes to get the checksum you want.
Good luck!