I'm afraid this is a no-answer, I'd like to post both a practical and a theoretical objection..
Where to find material to build your ship, it will fly away from you..
Suppose for now, negative mass actually exists. A galaxy could consist entirely of negative mass.. you'd only notice the void around it, it will look the same. In that galaxy, the negative mass would attract other negative mass, nothing would really change in the observation.
To let your propulsion work, you'd need both types of mass, you'd only have a ship built of negative mass, the rest of the world would have to be positive mass. Suppose it works (hand-waive hand waive) with a negative ship mass, how would you build that ship, in the first place.. I wonder where the building material for the ship is harvested ? You can't have a negative mass planet in your positive mass solar system. This planet would not stay near the sun, it would be in acceleration, moving 1000s of km/second away from you. Same with comets and meteorites. They won't impact on your positive mass planet, when their mass is negative.
Double-sided space time ? a second antipode reality ?
According to Einstein, the slope of a vector in space time will yield the perceived gravity at any point. The attractor resides somewhere "down", everything falling in the direction, along the gradient. We visualize heavy things as a "steep hole" in a 3d plane, the bottom being a stable point.
Now suppose you would model negative mass as the opposite: a steep hill in space time. Everything that sits on top of that hill will be unstable, prefer to move away from it, in any downward direction ! The ship will not move away from the planet, it will move toward the planet with even larger velocity. Conclusion: a steep hill does not work, to model your ship.
Instead, your ship would be on the opposite side of space time. You need a mechanism to climb up hill in our space time.. That would only exist, if our space time has an opposite, mirrored reality, on the other side, where the same gradient is perceived as go down hill. How separate would these two realities be ? And suppose you reside inside the ship, are you still in your world, or did you enter the antipode side of space time..