This sounds pretty cool, and I like the aspect of other players building ships for others. Sounds like a fun local multiplayer game to me!
But the shooting aspect seems overdone, like others have said. Mario kart aspects such as racing around tracks could be fun, and you could even be able to collect parts while racing, ex. A more powerful rocket launcher on the side of a road, or exclusive weapons only obtainable while racing. Maybe you could even steal other players ship parts while racing!
This is a really cool concept, and I'm happy to give feedback.