You may be better looking at second hand trailers if you want a Hi-ab with a donkey engine on it. Then scrap or sell the trailer after you have removed the crane.
One of customers has recently purchased an ex Travis Perkins 18 tonner and removed the crane with block grab & rotator and fitted a cement mixer in it’s place (not a readymix drum) running off the PTO. Last time I went to their yard it was sitting there exactly as it had been removed. I’ve no idea if they still have it, want it or are looking to sell it.
It’d be worth giving them a try, ask for Clint or Robbie.