frankly you would be better off changing out the speakers all together.
I did a friends 500sl (early 90's model)
Changed the headunit, put in amplifier, and put in components in the front and I believe a set of 4" as well in the front and 6x9's in the rear bench part, and it came out sounding pretty nice.
Thats how he wanted it. I didn;t want to do the rear bench, but he wanted more sound.
As for you, I would suggest you try some salvage yards and see if they have some. There are a few that may be able to get you these units used. I would check with potomac german auto or
www.silverstarrecycling.com
Otherwise you could try a set of components speakers without changing the bose amps and such and see if it works out. But I am not sure on the Ohm ratings of the bose setup or power specifications.
Regards,
Alon