If wanting to index Shimano 7-speed with a freewheel, I'd buy brand new, all-chrome 7-speed Sunrace from eBay, like this one:
https://www.ebay.com/itm/175249444991
They come in 13-25 and 13/14-28.
The all-chrome offerings are better quality (ime).
You may need to re-space the hub to fit the wider cluster. My 7-speeds are usually 1-2mm more than 126.