07 License plate number

Hello together, it is possible to allow for a 07 registration (there are already 2 registration plates) a small motorcycle (also 30 years and rebuilt) – i.e. 50ccm. There are red registration plates for mopeds – but I would like to keep the existing red number. Unfortunately, I could not help with a demand at the registration office. Please give me a tip where (§ STVZO) I can get in touch. Thank you very much.