Shimano MF Z012 replacement Freewheel?
Really any 6 speed freewheel will work. You'll want to get the tooth numbers close, but they don't have to be exact (your 14-28 will be a fine replacement.) Chain sizing for 6-7-8 speed drivetrains hasn't been significantly changed since they became popular, though the profiles and faces of the freewheel cog teeth have, in most brands, been altered to improve shifting quality (see the image below.) There is no need to worry about the tooth profile being incompatible with a modern 6-7-8 speed chain. 

You will, however, want to replace the chain. This is because as a chain ages and wears, it becomes longer. The chain rollers/bushings/pins are spaced at an exact 1/2", and as it lengthens, they wear the chainrings and freewheel cogs into a hook profile. If you use your old chain on your new freewheel, you'll likely find that it skips when pedaling (hopefully your chainrings aren't similarly worn, but if they are you will know what to replace next.)