Hi,
Yes I think you are going to need a "Universal Offset feedhorn" and a C120 LNB. It doesn't work out very cheap though.I think I would try and bodge something first, by making/finding a new holder. You may be able to buy a single holder online from somewhere, or try a 1"1/2 plumbing waste pipe clip.
Another option apart from an amplifier, would be to use better specified cable, such as wf165, the attenuation for this is only 20.9dB per 100 metre compared to 32.5 for wf100.
Following on from satmans calcs:-
80-
13.5-2 -2 =62.5 dBuV, a bit more of a margin for you.
Or in other words, the attenuation of 65m of wf100 is approx equal to 40m of wf100, which you should be able to get away with.
Robbo