Geert,
You should consider the mast bend and mast charactoristics also in your search for a new sail. The sail is generally tailor made for the mast and the way the mast bends when down haul is applied and how is behaves when the mast is rotated. The luff of the sail is also curved to match the curve of the mast when it is prebent due to the diamond wire tension on the spreaders. So an improperly cut sail on the luff will cause sail shape issues when the main sheet is tensioned. The type of mast section, the spreader location and diamond tension(prebend), and the rigging point for the shrouds and forestays also will effect how the mast behaves and the shape of the sail.

I would take a string and measure the mast bend along the luff track of the mast with the diamonds tensioned the way you sail the boat. Send this to your sail maker and get his input. His advise is most likely free. See if he will give you his design measurements and then compare this to whatever used sail you are considering.

Again, I think a used sail from another boat would be a mistake. If you could get it to work, it will be slow. A used sail from a 18sq with a similiar mast would be your best bet.

A boom on a catamaran allows you to adjust the outhaul, which controls how full the sail is, and is one of the controls that controls how much power the sail has. When its blowing you can apply lots of tension to the outhaul and it does a nice job of flattening the sail and depowering the sail, so the boat becomes easier to manage in heavier air. It also gives you more adjustment in the amount of twist the sail has, so you can have more outhaul and travel the boom down, but still keep the sheet tension high, this is effective for going to weather in windier conditions or footing when its windier. A boom gives you more options on how to adjust your sail in differient conditions. The boat will be easier to sail once you get all this worked out.

I would consider a new sail and a boom. It will be like having a new boat. Plus you will probably end up with a modern shape sail, which should be a nice improvement.
A rigging shop should have round aluminum sections that you could easily make into a boom. They would probably rig it for you too.

Your sail is your engine, get a good one.

Bill