Hello!
Based on the twin shock BMW, Boge are not rebuildable, as far as i know. They are the OEM brand.
It is funny you ask because we discussed shocks a few posts ago.
If yu want a new one, except the OEM there are EMC, Shock Factory and Fournales. The later are without a spring because they use air to replace it. This has two main advantages : As the pressure builds up on compression they become stiffer and, thus, can't go hit the mechanical stop. Second advantage is that you can change "the spring" by changing the air pressure inside to allow for a passenger or a heavy load. EMC, also French, is very good and cheaper and Shock Factory, owned by a Englishman is in France too and the cheapest. All three brands have very good reputation, are rebuildable and in the case of the Fournales will outlast the bike...
You can also go the South European route and ask if Betor does something for your bike (should be very cheap) or go north and have a look at Ohlins or White Power... In this case be prepared to trade in your last born child to pay for the shock...
