You can start your Suntour freewheel journey here :
http://www.yellowjersey.org/stfw.html
I've only changed cogs on the Winner series freewheels, so I can't vouch for interchangeability between the Winner and Pro compe series. A pair of chainwhips with some huffing and puffing will work fine. It's kind of like using a pair of scissors, so if you do need to use an extension, you'd need two.
It's hard to find ST cogs nowadays as stock is running out. Have you tried looking on the Loose Screws site? perhaps some forum member will have what you need too. Otherwise, it's much, much simpler to buy a new freewheel.