2.5 ford transit Diesel?

I recently purchased a 1994 Transit Diesel. Problem is that there is a severe lack of power from the engine? The van never carries much weight in the back so i am curiouos about the lack of power. The van was lying for about 6 months and the battery was flat. Lack of power seems to be when it is cold and the max speed is very low.??? any ideas as to what could be causing this?