Headlight plastics do tend to deteriorate over time which can cause a reduction in light output, but we would suggest a sensible first step would be to try upgraded bulbs such as Osram Night Breakers or Phillips RacingVision to see if this improves the performance. If this doesn't solve the issue then you could try a headlight restoring kit - 3M does a good one for under £20 - of you could find someone to do this professionally for you. If that doesn't solve the issue it might be a matter of either replacing the key components or the whole unit - the former might be a complex task for the home mechanic whereas replacing the whole unit with a good quality used or new part would be more straightforward if potential more expensive.