Have you measured the size you want, ground-to-ground? Click here for diagram for a full awning.
Please show photo of roof design before we suggest porch awning as the awning rail height may be too low for a standard one. We had this problem on an older caravan.
Due to the age of the caravan it would now be regarded as a classic. I would try http://www.period-classic-caravan-club.co.uk/ to see if they can help as this could need specialist advice. You might even want to join.