Some parts that you have spent a lot of money, (and time), fitting can make you think the car is worth more, but such as the windscreen, you wouldn't buy a car without a windscreen, the head, again time and money goes into replacing it, but a Discovery without a head. Running any vehicle cost money and most parts are consumables after a set time or mileage.
You have 3 options, first sell the parts which may result in more money than the car is worth but over a long period. Second, sell the Land Rover as is for not much money, (just look around at Discos with 12 month ticket for less than £800.00). And your last choice, weld the car up, new mot and get the price you think it's worth.