Fortnite already has a game mode in it called "rocket racing" which is a pretty competant track racing game.

I'm guessing it'll be like their other game modes where they basically port it to the specific version of Unreal Engine that Fortnite uses and build it in as a gamemode. There's actually already some clones of Fall Guys and Rocket League in Fortnite but it's still more a case of "playing $realGame but worse" due to limitations of the exposed engine features