You need a hydrogen 'scoop' and fusion. If you can accelerate to 0.01c, you can collect enough hydrogen for 1000 MW fusion power from 32 km2 (6 km diameter) scoop area. Faster you fly, more you get. You can accelerate until interstellar particles start to do real damage.
> we estimate the neutral hydrogen density in the unperturbed local interstellar medium of 0.195 ± 0.033 cm−3 https://iopscience.iop.org/article/10.3847/1538-4357/abb80a
You need a hydrogen 'scoop' and fusion. If you can accelerate to 0.01c, you can collect enough hydrogen for 1000 MW fusion power from 32 km2 (6 km diameter) scoop area. Faster you fly, more you get. You can accelerate until interstellar particles start to do real damage.