Originally Posted by BC6260

After revisting Sheldon Brown's tutorial, I have definitively determined via the "spin test" that the MF-HG37 is indeed a freewheel. The removable sprockets on the splined body had me believing otherwise, but it looks a lot like the Shimano Hyperglide Freewheel photo from the tutorial. The lock ring I removed is the one the photo caption says "it is never necessary to remove".

So, the Shimano MF-TZ21 14-28t looks to me like a good replacement, as does the Sunrace 13-28t you mentioned.

Thanks again for your help. If you're interested in seeing the photos of my disassembled freewheel, I'd be happy to keep chatting until I reach 10 posts.