f1gstate.sav checksum problem after modifying the file

Before to start a GP4 quickrace I'd like to modify the Quickrace Gridposition using a random value. I succesfully created a script that use XVI32 to edit the f1gstate.sav file.

But after the f1gstate editing with XVI32 when I start GP4 I get the following messages.

Unable to restore saved state.
Bad or missing f1gstate.sav file.

I suppose the problem is that I also need to change the f1gstate.sav checksum.

Can someone suggest me how to modify the f1gstate.sav checksum ?

Thanks in advance,