Hmm, the part number MF-HG37 matches up to a freewheel. Is that what you have? Here is a reference:
https://www.sheldonbrown.com/free-k7.html
...and a pic from the reference below. If you really need a freewheel, then you can search eBay for "shimano tourney freehwheel 13-28" and you should get some relevant results.