Replacing rusted part of gutter

[vg_39]

Hi 

I have the elbow joint part of the gutter (see attached pics) which needs replacement as it’s fully rusted. Can someone advise the next course of action. The dimensions of the pipe is 100 *75. I have found this pipe in Bunnings which will do the job however I can’t see any elbow joint pipe. The dimensions of elbow are marked in the pic too.

Any help and options on how to progress from here will be appreciated. Cheers