You can update the rom so the checksum stays 00 even after editing it. Here is how to (using Win Hex in this example, but you can use the program posted below, but only to find checksum, not to edit the ROM) :

Open Win Hex and select Tools Menu -> Calculate Hash (Select 8 bit). Write down the number it gives you. This is the current Checksum.

Open Windows Calculator (Start -> Run -> Calc then press enter) View -> Scientific Select Hex Mode (or press F5)

Do this : FF - Current_checksum ( FF - AB for example) The number it gives you is tu be written in a memory location, toward the bottom, where no code is present, and where the value FF is already there.

Example : (FF - AB = 54)

lets say I go to memory location 7FFF, value is FF. I replace FF by 54.

if you dont have an address with a free FF value, but another value (lets say 00), use 00 in calc instead (00 - AB) and replace the free 00 with the result.