No replacement module. Consider connectivity and features on the Infiniivision 5000 series oscilloscopes.
The Keysight 54659B RS-232 & Parallel Measurement Storage Module is compatible with Keysight 54600B-series scopes and the Keysight 54645A/D. The Keysight 54659B can be used with the Keysight 54620A/C logic analyzers for I/0 only.
Need to analyze signals via RS-232 or parallel in the frequency domain with FFT? Need more nonvolative waveform memory? This is the module you need for your Keysight 54600-series scope.