Good to know that there are still a few ELP fans around!
The intro horn is definitely a Yamaha GX1, as are all of the synth sounds used on Fanfare for the Common Man. Keith used three registrations (patches,) the intro horn, the brass like poly synth lead, and a modulated organ-like lead; I believe that that synth bass sound was simply the lower octaves of the organ-like patch. The Moog was not used at all, except occasionally on extended live versions that formed a finale. I know this because I discussed it with Keith and his keyboard engineer, Will Alexander when I met with them a couple of years ago.
We produced some Fanfare patches for our Superwave Performer which do sound credible when played live. Whilst they are not indentical, they probably as good as can be achieved without a GX1, especially, with a software synth. You can hear the sounds by clicking on the link below.
Regards to all,