Last year I installed the Klima Design works monovalve replacement kit on my 560SEC.
It wasn't a drop in replacement, but worked well and was a good option given the Bosch rebuild kits for the monovalve are NLA.
I've just seen they have updated their kit with a true drop in replacement.
klimakit.com
I would have used this kit, had it been available early last year.
I am also interested in their paralell...