Do you have indexed or friction shifting?

Freqently you can squeeze 7 speed freewheels onto your hub with only minor mods.

DNP Epoch makes an 11-32, 7 speed freewheel:
DNP Epoch Freewheel 7spd 11-32 Nickel Plated

It does have a pretty big jump up to the 32 though.
Freewheel Cogs: 11,13,15,18,21,24,32

One thing about the DNP freewheels is that they have lot of back spacing, so they may be problematic.

With the 11T, you can go to smaller chainrings if you desire.

Also consider the Shimano MegaRange freewheels (although with big jumps).