I don't know if this is some sort of difference between British English and US English? Does "fathead" mean the same thing over there?
come to that, how does this term originate? Flatheads, yes, widely used term which has been around a long while and widely used outside the Harley lexicon. Knucklehead, Panhead, Shovelhead, well-known and generally accepted terms current then and since.
"Evo" seems generally accepted and to some extent, "blockhead" - although I've no recollection of seing that one outside the "custom bike" context and by no means universally applied there either. "Twinkie" or "Twink" seems to be another term occasionally used, although probably not outside the US for the same reason that Terry Pratchett's joke about "Djelibeybi" doesn't work in the US
but "fathead"? Oh, surely not...