I'm sure someone like Roose will make a pipe for this conversion that doesn't involve bodging together different pipe types. Its a common enough swap that someone will have created a standard part for it.
Regardless, with what you've got -
That joiner looks like a mess. Those welds don't inspire confidence there isn't plenty of pinpricks/porosity in the joiner itself. Is it definitely the pipe leaking and not a seam in the joiner itself?
Does it have beads to help seal the pipe to the joiner or is it just plain slip on? How far "into" the pipe are the joins going as well