OK... so you have Audio20 NTG4.5, not COMAND NTG4.5.
The designation 'NTG4.5' applies to both, although Audio20 and COMAND are two different system. Sorry for the confusion.
The Becker Map Pilot that came with your car will only work with the Mercedes-Benz Audio20 unit, it will not work with any aftermarket unit.
So you could:
Repair the Audio20 with the faulty optical drive
or
Replace the faulty Audio20 with a second-hand unit off eBay
or
Buy an aftermarket infotainment system without navigation, and use a standalone SatNav on the dash (TomTom / Garmin etc) or a smartphone-based app (Google Maps / Waze etc)
or
Buy an aftermarket infotainment system with built-in SatNav (branded e.g. Alpine/Pioneer, or generic Android-based)
Hope this helps.
PS - if you do replace the Audio20 with an aftermarket infotainment system, that you can sell the Becker Map Pilot unit on eBay or via the Classified section on this site. They go second-hand for £100-£200 (depending on the model and maps update version).