What age 90 do you have? Most have sealed beam units, although they did change to halogen at some point post 300Tdi.
If it's sealed beam units, then you'll want to buy some halogen lamps (loads about, some better than others, the newer style ones with clear glass and angled reflectors emit more light than smooth reflectors and angle lens glass).
And then some better bulbs.
If you already have halogen lamps you only need the better bulbs.
Lots of choices with bulbs, standard is 55/60w.
Now you could go for some of these 80 or 90% brighter bulbs at the same wattage. Halfords sell their own (usually BOGOF), but Phillips and others also make such bulbs.
Or you could go for some higher wattage bulbs. Halfords sell some "Rally" bulbs in bright red boxes (often you'll need to order them in as stores don't always carry them).
As a rule they are 80/100, 80/120 or 90/130w. They will all be brighter than standard bulbs, but will run hotter. Cheap halogen lights might not like the 90/130's and it'll smoke the glass due to heat. You might also need to adjust the lights down a bit so you don't blind people.
"Technically" higher wattage bulbs are not legal, however I don't think anyone has ever been prosecuted for running them. They also will still be putting out less lumens than modern HIDs, so won't run the risk of dazzling people anymore than new vehicles do.
Personally I'd stay away from any "blue" or "cool" tinted halogens. Usually they look horrid and blue light causes more eye strain and will wash out colours, making definition of objects at distance more difficult.
Next up from this would be to consider some aftermarket HIDs. There is currently a huge grey area on their legality. At present to my knowledge there is nothing to say they are actually "illegal". There are many opinions, even of government departments, but none of this is actual legislation.
Therefore you run them at your own risk, although as with higher wattage halogen bulbs I don't think anyone has been prosecuted for HIDs.
With a Land Rover as the single light unit does dip and mainbeam you'll need some Bi-xenon HIDs. These are a little cover that moves to make them go from dip to main beam inside the bulb unit.
If you do go down the HIF route please please please only go for 4100-4300K rating. These are "daylight" white, offer up the most lumen out put and the most minimal glare. Higher K ratings will have a horrid colour cast and tint to them. 6000k is blue white and very cool, again this causes eye strain and washes colours out. These also put out less light than a 4300K HID. So there is no reason to run them.
I'm not certain on bi-xenon HIDs, but I think there are two types, one for "projector" style lamps and one for "reflector" style lamps. The latter has an additional metal strip on the bulb that reduces glare. Fit the wrong type of bulb and all you'll do is **** off other cars.
With HIDs you will likely need to adjust your lights, you will also need to check for scatter. The HID element will likely be located in a slightly different place to the halogen one, so this affects how the light is reflected out the front of the unit. With some research and wise purchase choices you should be able to massively improve your lights without annoying other road users however.