Wren 54 MkIII. You can even build the engine yourself at the factory during one of the factory open days and save some extra money and learn about your turbine at the same time. The kit is also available to build at home but I would recommend building it at the factory and make use of the free support. If you don't want to build the engine yourself then you can buy one ready built. The Wren customer service is excellent and the 54mkII now gives 15lb thrust and powers the boomerang just fine. I have several engines from the Wren range and use a 54 in my boomerang.
http://wrenturbines.co.uk/