You may find machine built 700c rim freewheel hub wheel sets
from the suppliers to your LBS.
easy enough to swap spacers to make it work, 120, 126, etc.

120 dropouts will also get you in the realm of a 3 speed IG hub.

fitting a 3/32 wide (2mm) cog on the hub
would let you use the 2 chainrings in the crank,
and the old RD to take up the slack..
It doesn't need a cable attached to it, then..
center it over the 1 cog with the limit screws