EXM 378-2: 6EM7 378-2AB00-0AC0

EXM 378-3: 6EM7 378-2AC00-0AC0

The EXM 378-2 and EXM 378-3 expansion modules are designed to carry interface submodules. By installing suitable interface submodules such as IF962-VGA and IF962-LPT in these expansion modules, you can connect, for example, a VGA monitor, a keyboard and a printer to your automation computer.

The serial interface COM1 (X1) is addressed according to the AT standard in the range 03F8H to 03FFH

There are further descriptions of the use on the FM 356 in the manual, shoeing how it can be used to map communication areas for fieldbus networks (MPI and Profibus-DP) to the AT standard used by the M7-300.
As for your second question, I mostly doubt that other modules are available to replace the EXM 378 and the FM 356. You should contact Siemens to see if there are available spare parts, or possibly look on the web for repair and refurbishing shops if you need to buy some replacements.
Hope this helps,
Daniel Chartier