FWIW I installed a 6-speed Shimano freewheel - MF-HG22 14-28t and it worked just fine - I haven't had it out on the road but was able to tune it well in all three front gears on the stand.
I was searching for a suitable Suntour freewheel but $40-$50 was the least expensive I could find and I didn't want to have something else laying around if it didn't work.