Tada, everything you ever wanted to know about the Forma Urbis. The subpage "Slab Mab" shows the fragments in their relative position, you click on the sectors shown there and it sends you to another page with all the identified fragments for that sector.

I'm not entirely sure what you mean by your other question though. Are you asking whether the ancient street plan is still followed by the modern city? If so, with a very few exceptions (such as the Via Sacra) no, not really. The modern city has largely been built on top of the ancient site

By the way, that fragment that you linked to doesn't show roads it shows buildings. The Forma Urbis indicates the walls of buildings with single lines, which would've been painted red in antiquity, and those little dots are columns. The spaces inbetween buildings are often alleyways or roads, but aren't necessarily, since some buildings are things like storehouses or market stalls between which there wasn't anything but empty space. Larger streets and plazas are often obvious to us because they're labelled