vent through roof problem

to make a long story short i have a 2 inch galvanized vent stack that comes up from the kitchen that comes into the attic and connects to a 2" to 4" enlarger than continues out the roof in 4inch cast iron


some how were the 2 x 4 connect to the 2" gal got broken so my questions:

1- is there any problem using PVC for the repair?

2- is it some sorta code that it must enlarge to 4" cast or can i just continue up with 2" and put in a 2" roof grommet?

thanx

No problem at all using pvc for the repair.

The code is that the section going through the roof must be larger to prevent frost closure in the winter.

thanx alot!