gearbox for HiJet pickup


I have just bought a R reg hijet pickup, with a camper van body dropped on it. The currennt 5 speed gearbox seems to be to be too highly geared for the 1.2 diesel engine. Consequently the top speed is only about 50MPH. Can you get a gearbox to slot in that will have lower gearing to give a higher top speed? The car seems to have loads of Torque so it can probably take it