There is no national flute registry that you can check. Your best bet is to post here (as you did) and also on several of the Facebook Flute groups. Maybe the person who bought it will see your postings someplace and contact you. Also keep an eye on eBay in case someone decides to sell it there.
Probably the next best thing is to contact lots of flute shops and try to find another Muramatsu DN with a serial number in the same general range as yours. Muramatsu America currently has several used DN models for sale. You should also try a DS model which is the current equivalent model that replaced the DN. You may have better luck with those options.
BTW - I understand your love of your Muramatsu flute. I play a Muramatsu AD and upgraded it with a Williams 14K white gold headjoint. I would not trade it for any other flute.