Originally Posted by MichaelW
You can replace a 5 speed freewheel with a 7speed one with no major modifications.
http://harriscyclery.net/site/showit...ory&Catalog=39
You may need a rear derailleur with a long cage. I find that any low-mid range shimano one works fine. Your existing shifter should work in friction mode with any mech/cog combination.
Cage length is about wrap capacity.It would be more appropriate to get one with a big cog capacity to shift a 34.