Official confirmation of a lost car registration

We are borrowing a Belgian car and lost the car registration. The Belgian authorities said we need a "loss declaration" by the police before they will issue a new one. Does the Polizei provide such a service? How would we get it? We would rather not wait until the car is pulled over and the missing registration is discussed at that point...

go to your local police station and tell them.you then get a piece of paper letting the belgian authorities know that the plates are being searched for in Germany.