This is what I've got so far:
The checksum function is located at 08074624, it takes input r0 = 0 when it's checking the save data. The format of the data at 02038000 is as follows:
02038000 + saveID*0x1220 + 80 is the start of a save slot's data, and saveID = 0,1 or 2. I'll refer to this offset as the SaveOffset. The game does 3 things to check if the save is valid.
1. The game adds all the words in the 0x1220 area and compares that to the value stored at [SaveOffset, 0x10]. These values should be equal in a valid save.
2. The game compares the words stored at [SaveOffset, 0x10] and [SaveOffset,0x14]. It checks if the words are bitflips of each other (i.e. [SaveOffset, 0x10] + [SaveOffset,0x14] = 0xFFFFFFFF)
3. There are 3 strings the game expects to find:
3a. The first string is at [SaveOffset,0], the game expects this string to be "ZERO_MISSION_010" (The exact byte sequence is the 16 bytes starting at 08411410.)
3b. The second string is at [SaveOffset,0x4F], the game expects this string to be "Planet Zebes" followed by the 4 bytes 0x2e, 0x2e, 0x2e, 0x20 (The exact sequence is at 08411420.)
3c. The third string is at [SaveOffset, 0x250], the game expects this string to be " - Samus Aran - " (The exact sequence is at 08411430.)
If none of the strings match, the game will ignore the save data. If there is a partial match, the game will report the save data as corrupt. If the strings match, but 1 or 2 fail, the game also reports the data as corrupt. Otherwise the game thinks the save data is valid.
I haven't checked how the game actually creates the checksum, but looking at the procedure, this should work:
1. Add all the words in the save slot except the words at [SaveOffset,0x10], [SaveOffset,0x14] and subtract 1 (Call this X.) Store X in [SaveOffset,0x10] and -1-X in [SaveOffset,0x14].
1. Store the strings in their locations.
Here's a partial disassembly of the routine: http://pastebin.com/6vEhYbrm
If anything's vague feel free to ask for clarification :p