I fitted a Blueflame stainless twin exit system to my ST24, but a good few years ago now (over 10 years ago). iirc at the time peeps said the Blueflame was actually same as the Mongoose system. It fitted perfectly, even the hangers were spot on, which surprised me! As you say may not make it anymore!

Becareful about performance, I went to rolling road and someone with an identical ST24 to mine with a new Ford OEM system on her ST24 got exactly same bhp figures as my new twin exit Blueflame system.
Didn't bother me cos I wanted twin exit which I think suits my car better and the larger pipes look good!

Another option might be MIJ exhausts, at the time I know alot Mondeo ST owners had exhausts custom fitted by them, and it got good reviews.