I ran that very derailer with an 8s, 11-28t cassette, and it worked surprisingly well.
So using a 7s freewheel having modern HG-type tooth features can give very good shifting across a 13-28t range which is currently offered by Sunrace.
You will likely need a longer chain, and I believe there may be an adjustment feature at the mounting bolt which will adjust the "B-tension" to accomodate the 28t cog.
You won't have a lot of leeway in terms of chain length with which to alter the chain gap between the pulley and largest cog, but adjusting the axle position in the dropout will possibly allow either some chain length choice or at least some fine-tuning of the chain gap.
Some of the chromed Sunrace freewheels are very good-looking, here's a 13-25t:
And here's a 13-28t made by Giang, axle spaced for only 124mm spacing:
