Would something like this work?

https://www.screwfix.com/p/mcalpine-...questid=108942

Connect it to the brass inside the cistern and then thread it through the hole and connect downpipe to it. Not sure how your setup seals the hole in the cistern but that would have to be done here too obviously.

I did a quick Google for Plumbing thread extenders which came up with a few variations on this.