[Legbreaker]

There doesn't really need to be a separate and specific set of rules for urban gaming - the basic rules do fine if properly applied.
Carrying a person can be done in several ways - firemans carry over the shoulders, or on a stretcher being two examples. In the former option, the encumbrance rules are sufficient - the carrier can handle up to their Load stat without penalty, up to twice Load and only move at a jog (at most), and up to three times their Load for short distances (a hundred metres or so at a time). Both of the latter two load levels count as hard work and fatigue sets in very quickly reducing the characters physical ability to do, well, anything really.

Moving through bombed out areas could involve an Agility, Climbing or Acrobatics skill check (may vary depending on rule version in use). Visibility is a GM judgement call, but you could use the encounter ranges for forests quite effectively.

Much of what you need to do is common sense really - buildings and rubble will impede vehicle travel almost completely while slowing pedestrians to about half normal move (as they weave in and out of doors and windows) if they're not out in and using the streets. Cover and concealment will be everywhere which can be used by both sides in an engagement to very good effect.

As for large scale movement, a foot patrol speed in light forest is about 5kph (the 20km per period of the rules). Marching speed on road with full gear is rarely as fast as 10kph (and you don't want to be doing that for more than a couple of hours). In jungle/close terrain, speed drops through the floor and can be as little as a few hundred metres in an hour. Unstable rubble is likely to have a similar restriction on movement AND as is mentioned in Ruins of Warsaw, possibly result in a very nasty accident.

As for your intention for the party to stay in Kalisz for several days, I just can't see that happening if they really want to get out. We're not talking a major metropolis here and unless they're hiding from roaming Soviet and Polish patrols, they'll be able to pick a direction and get out fairly quickly. Getting lost would be nearly impossible unless forced to move under cover of a moonless and cloud covered night sky (even then it would be a bit of an effort if even one character has any navigation skill to speak of). There's any number of ways to slow a group down (a wounded member who needs to be carried for example), getting lost isn't one of them, although encountering rubble/enemy/gas attack/fire/artillery bombardment could force a rethink of their intended path.