Replacement for Suntour Superbe Tech Rear Derailleur
You have friction shifters so you don't need to worry about shifter compatibility -- any derailleur can be shifted with a friction shifter provided the shifter pulls enough cable to get through the range of required shifts. So, all you need to worry about is the capacity and extreme cog sizes. 

I'd just buy some Shimano derailleur like a Shimano Acera one for 15-20 bucks.