Lev,
Best wishes for your journey with the Aceca Bristol. Not sure where you are, but in the UK I believe the best people for special radiators, oil or water, are Serck Competitions department at Park Royal.
One route to an original water radiator is to start with a 2 litre saloon radiator. They are basically an identical unit, but on the saloon incorporate a a flange that seals the space around the radiator. This flange can be removed to make it like the Aceca or Ace. Be aware that the later Ace/Aceca and Ace/Aceca 2.6 used a Greyhound pattern radiator which had a more rectangular header tank.
You will find many photographs in various publications, for example Rinsey Mills' 'A.C. Six Cylinder Sports Cars.'
Barrie