I have just purchased an automatic 2007 crewman running the alloytec engine, I intend to use it to tow a small van (900kg). From what I have read these motors lack torque for towing or torque is developed too high in the rev range for the gearing it comes

A recalibrated PCM would help with your requirements as shown in the link below:

http://www.maceengineering.com.au/epages/shop.sf/en_AU/?ObjectPath=/Shops/mace/Products/COMP621

However, a much better option to start with would be either our 12 or 25mm manifold insulator as shown in the links below. The latter would certainly be more effective given your application.

COLD AIR INTAKE MANIFOLD INSULATOR ALLOYTEC VZ-VE

http://www.maceengineering.com.au/epages/shop.sf/en_AU/?ObjectPath=/Shops/mace/Products/MI603

25MM COLD AIR INTAKE MANIFOLD INSULATOR ALLOYTEC VZ-VE.

http://www.maceengineering.com.au/epages/shop.sf/en_AU/?ObjectPath=/Shops/mace/Products/MI603