I had the same problem. The plastic seat was totally busted.
After contacting Amel, they told me they cannot supply a replacement, so I looked on line.
I found a really good replacement at Wayfair in the US.
I bought 2 different styles called "Clay Stool by Wade Logan" just to have a spare.
You have to redrill the existing holes, but it is very easy to adapt and works just like the original.
No one would know it is not the original - maybe not quite as comfortable, but...it works and it is cheap.
Phantom A54 #44