You might want to consider a 7 speed Shimano HyperGlide freewheel. It will probably fit in the 126mm 5 speed spacing. You might be able to find a Mega 7 freewheel described in this article.

I replaced a Suntour freewheel (New Winner, I think) with a 13x28 Shimano HG last year. The HG shifts better because of the improved tooth design and the addition of ramps. And the new body design has noticeably less wobble.