There are no less than three different types of Beovox S45.
The woofer in Beovox S45 Type 6427 (two-way system) will suffer from foam rot if not already refoam'ed.
This woofer has B&O part no. 8480158, it's a 6,4 Ohm woofer unique to the Beovox S45 and S55.
The woofers are repairable and I have a pair for sale, refoam'ed and ready to mount.
The woofer in Beovox S45 Type 6302 and Beovox S45-2 Type 6312 (both three-way systems) is a Peerless with B&O part no. 8480114 and is unique for those models. It has a rubber surround so will not rot but the magnet tends to losen and the coil will be locked.
The woofer in Beovox S40 (and Beovox S35-2 plus the latest revision of Beovox S35) is a Philips AD 80652 W4 (or AD 80651 W4) with B&O part no. 8480135.
This woofer can have either rubber or foam surround, both types are found.
They are all 8 inch woofers, but that's about as far as they compare.
The rest of their T/S values are completely different so they will not be interchangeable without drastically changing the sound reproduction.
I haven't found any modern replacements that comes just close.
Martin