A while ago I managed to repair a Lenz Gold Maxi by replacing the FET output chip that had obviously blown - fortunately I could identify and source the component from Conrad. As Mike (Dutchlem) says, you need to be handy with micro soldering.
Although it worked, the decoder was never quite the same again (probably some other component had suffered and was on the verge of failure) so I withdrew it. To be honest I tried the repair more as a challenge: it was significantly cheaper than buying a new decoder, but fiddly. Don't think my eyes would really want to try it again!
If it were me, I'd just source an alternative decoder - probably another cheaper brand unless there's a compelling reason to continue with LGB/Massoth because of a unique feature you need. Depends which LGB decoder it is I guess?