Which lnb should I think of buying one.I don't think its possible unless you have some specific microwave test equipment, knowledge and skills to do the mod. It would require changing the LO frequency and retuning the front end filter. It would certainly cost more in labor than a new extended LNB would cost.
I have had a good experience with Swedish Microwave which should be easy to purchase in EU. Here is a link to their C-band LNBs.Which lnb should I think of buying one.
Live in Holland.
Best regards