You cannot choose one. No off-the-shelf electronic crossover can replace the built-in one. To do this job, the crossover needs to be custom designed for the speaker and this is not a task for a beginner.
I suggest that, for the moment, you leave it as-is and pick up a book on speaker design, e.g., Dickason's Loudspeaker Cookbook.
Kal
I suggest that, for the moment, you leave it as-is and pick up a book on speaker design, e.g., Dickason's Loudspeaker Cookbook.
Kal