There’s a Suzuki mouthpiece kit that has sometimes been sold by Rakuten Global and also on eBay for around $45, Lee. It includes several shapes and sizes, any of which should fit the Hammond.
But the mouth hole fitting from any current Suzuki melodion tube will also work. I just checked one with my Hammond 44. I sometimes use this as a short mouthpiece with Suzukis and like it, partly because of the small size of the opening into which you blow.
Ah, I just found this. It’s for the Pro-37v2, but basically the same as for the Hammond:
https://goldharps.com/suzuki-pro37v2-melodica-short-mouthpiece.html