Also... I'm intrigued by the concept of multiple 'bots wearing the Magnus armour so I decided to lean into that.
Kingdom UM and CW UM were pretty easy. Each is distinct and each has a distinct guy in the armour. Re-purposing Siege UM would only really work to my OCD self if I made the inner robot as distinct from the white Optimus inside Kingdom UM as possible. So I left the crotch plate up in armoured configuration and swapped out the head for...Classics Jetfire's head (still on theme!)
I love that Classics Jetfire toy and I couldn't bring myself to get rid of it when I replaced it in my collection with the Japanese release of the Generations Leader, so I kept it in storage. But it wasn't doing anything and I realized that it was just the right shade of Hasbro off-white to use on the Siege UM body! So there you go.
Kingdom UM- Original Ultra Magnus
Siege UM- Datum
CW UM- Minimus Ambus