Each savegame section has its own simple 8 bit checksum. Instead of byte swapping and hoping, you can enter the value for the item you want, select the entire section representing the save, calculate the 8 bit checksum and edit the checksum byte. For example, in my save.bin, save 1 starts at 0080 (the second instance of "03") and ends at 3BDA, with the checksum at 3BDB. I can highlight everything from 0080 to 3BDA (inclusive) and calculate 8 bit checksum, then edit 3BDB to match and save the file.
Many hex editors have checksum and CRC calculators built in. I find it easiest to use ADB to pull the save.bin and edit it on my pc, then push it back onto my device. If you are having difficulty finding the save.bin file, your file manager is probably looking on the sd card instead of the internal memory.
