How near to 2050 is it? How far behind is your score? Unless there are only a few turns left then your first priority should be to stop anyone else from winning by another method. Check the victory screen (the red fist button in the top-right corner).
If anyone is nearing a Culture victory (if they have three cities that are over half way to Legendary culture then chances are they are going this way) then you need to raze one of these cities ASAP. You don't need to wipe out the civ - taking one big culture city should be enough to take them out of the game. Check the victory screen again though because they could have other cities producing a lot of culture.
Similarly, if any civ is nearing a space victory, you will have to take out any high production cities they have. Use spies to check which cities are building SS parts. Obviously, going to war this late in the game is difficult, but it might be your only option.
Razing the UN will guarantee that no one else will win by Diplomacy. If you have enough votes though, this shouldn't be an issue.
Once this is taken care of, or if no one else is anywhere near a victory condition, you might wish to raze some of the score leader's border cities. This reduces their land and therefore there score too. If it's not too near 2050 there might still be time to build the spaceship once the competition is out of the way.
If anyone is nearing Domination, chances are you've already lost
.
Summary: To win a time victory, first you need to make sure no one else wins another victory type. After that, it may be more practical to reduce the other civs' scores rather than increase yours.
Good luck
I'm in a sticky game right now, where the only solution is score. There's too many civs for war, it's late in the game, space seems impossible, and I'm not loved as much as I would like. I'm behind in score, but I need to increase it.
