The time to think of replacements has long passed, not to fly by the outer planets, but to achieve a greater velocity than the Voyagers and continue exploring the interstellar medium.
Some other comments here mentioned the tech to do that:
- ion propulsion
- light sail
And also nuclear fission might be an option, I like the fission fragment idea for its simplicity. [1][2]
[1] https://www.nasa.gov/general/aerogel-core-fission-fragment-r...