I'm not sure if they have a size to fit the factory openings in your car, but I would recommend MB Quart equipment as a replacement. They are German made, and sound wonderful. Although they are a bit pricey, they're worth it for the excellent sound reproduction. Go to www.mbquart.com
and check out their website for info on the different types of systems they offer. I have Boston Acoustics and Clarion speakers in my 6.9 along with an Alpine head unit. The whole system sounds great (I didn't install it--the PO did) except I found the receipt, and for just that equipment, the bill was a whopping $1,400!! So keep the Becker head unit, and just replace the speakers. I am going to put MB Quarts in my W-116 300SD next summer but retain the original Becker Mexico. Hope this helps!
'67 250SE Cabriolet
'93 300E 2.8