Hi Chia,
I have heard only good stories about John Van Stelton in the USA (
http://www.focalpointlens.com/fp_intro.html) repairing and even completely overhauling lenses.
Also Leica service in Solms might be of help if they still have the elements in stock. A friend of mine just had his 50/2DR serviced by them at Solms and it came back perfect with both new front and rear elements. Not cheap, but perfect.
Anyway, if the lens elements are OK (no scratches, no damage to the coatings, no etching by fungi), I am pretty confident that almost any skilled Leica repairman can clean inner fog. Sadly, there's no way to tell if the internal coatings are damaged or not from the outside and before attempting to clean them. As a side note, the manufacturing factory tolerances in Leica lenses are so tight that they rarely need realignment, collimation or re-centering at the reassembly stage (not an invitation to try this at home, though).
best,
Ed