Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your decking repair project.
I've just been speaking to one of our stores concerning your required decking panel sizes. I'm so sorry but they are currently not available. However, you mentioned that you will be needing 4 pieces to repair your deck. I recommend using 90 x 22mm 4.8m Decking H3 Treated Pine. It should be enough to cover the space you are trying to repair. I suggest using the Treated Pine Outdoor Timber Framing 140 x 45mm for your slopping board. The store will be able to cut your decking panels to 1.5m. However, the saws are only able to do straight cuts not complex or angled miters. You will have to use your own circular saw to cut your slope.
Let me tag our experienced members @tom_builds and @JDE for their recommendations.
If you need further assistance, please let us know.