I was intrigued and did a bit of searching. There are a couple of kits for changing the 900 to cartridge (i.e. not just emulators) from Andreani and Matris. They run around the £500 mark. Maybe there are others.
I have had a lot of work done by Maxton, they are world class and in Cheshire, they manufacture suspension to your desired usage and weight. I cannot recommend them highly enough ( and no I don't have shares in Maxton )
they did a set of SD25 cartridges for the TDM and an RT10 rear shock, the result is absolutely sublime, I cant think of any other description.
I have had Ohlin's shocks in the past and they are an improvement for most bikes, BUT they are not set up with exactly the ride spring rate, damping and preload.
hope it helps you make an informed decision
Howard