You don't need any specific model, the majority of derailleurs ever made will work for that bike. The only concerns when replacing friction shift deralleurs are whether the derailleur supports whatever size your largest rear cog is (probably a 28 tooth) and that it's total capacity is at least (teeth on big rear cog - teeth on smallest rear cog) + (teeth on big front chainring - teeth on small front chainring). For example, if your freewheel is a 14-28T cluster (pretty typical for bikes of that vintage), your big front chainring is 52 teeth, and your small front chainring is 39 teeth, you need a derailleur that has a max capacity of at least (28-14)+(52-39) = 27.

To simplify this, any derailleur designed for a mountain bike will work, as they'll be designed for large rear cogs and large total capacities. Mountain bike derailleurs include Shimano's Altus, Acera, Alivio, STX, Deore LX/DX/XT, and Suntour's XC series, among several others.