how can I fix the reverse on my Mountfield 5 hp 2 speed rotovator?

I have recently acquired a Mountfield 5 hp 2 speed and reverse petrol rotovator with a Briggs and Stratton engine. I have the operating instructions but still can't get the reverse. It goes eeeeeeee and then clunk and the engine cuts out. It is a fairly old model, but has hardly been used as the paint by the motor has not been burn't off.