As far as i know, there is know way , by now, to recalc the games checksum. So your game will be damaged and unloadable if you change a bit.
You can try to disable the filecheck in the game ececutable ( i gave up after 2 hours).
I cheated nontheless. if you have the xbox sdk and a debug bios on your machine you can use this to alter the memory of the xbox, while the game is running.
I added some lines of code to the xbox debug console example (part of the SDK), that copies the whole 64mb of the xbox mem to a file on my pc. There i'am able to search for what i want to change. Then i use the MS devstudio debugger to debug the running app and change the according memory locations on the xbox.
i cheated myself some billons of credits this way.