What your going to have to do is to support the new section of piping with a riser clamp or if your going to use plastic, support the stack up to the roof with a riser clamp. This will hold the weight and not affect the horizontal section. A riser clamp is two steel bars that are bent in the middle to circle the pipe. At each end where the pipes rest against each other there is a hole with a 3/8 inch bold. As you begin to tighten up the two bolts at the end of the clamps, they tighten up around the pipe. Once the clamps are tightened, you screw 2 - 3/8 inch lag screws up between the vertical bars into the wood. A fender washer holds the lag screws, so the heads do not slide between the bars. This will support the cast iron pipe up through the roof. Once the clamp is in place, you can cut out the horizontal section and replace it with anything you would like. I would suggest using a mission or fernco stainless steel banded transition coupling, to go between the cast iron and the plastic (and or cast iron no hub and cast iron hubbed service weight pipe).