If you are shifting in Suntour index mode, you need to match up the spacing.

Typical Shimano 7 speed UG/HG freewheels and cassettes are spaced at 5mm, (cog thickness 1.85mm + spacer thickness of 3.15).

First thing is to measure your current Suntour freewheel spacing. I think some are spaced at 5mm and others are split spaced. Three are at 4.8mm and 3 at 5.0mm. You might still be able to fudge it, but you might not get it to work well.

If you are friction shifting, any freewheel will work.

John