The seadoo number kit for the sparks comes with blank white sheets of adhesive that is made of the same stuff as the numbers. Cut out a square the size of your reg sticker, and stick the reg stickers to that.
Here are the officially BRP endorsed registration number stickers. Digitally printed on engineering grade high-tensile vinyl with aggressive, permanent pressure-sensitive adhesive. The special adhesive system is designed to adhere to “hard-to-stick” and low energy surfaces.