Hi @dadeaka ,
It looks like the upper case will have to be replaced.
Here is a link to the ifixit [guide|4789] guide, which should be of some help.
Here is a link to a [http://www.powerbookmedic.com/Nintendo-DS-Lite-Parts-for-Repair-and-Replacement-p-1-c-631.html|supplier] of parts for the device. (scroll to the bottom to find upper case parts). It is not a recommendation to use them. It is shown to give an idea of the cost of the part.