I did it way back when. This was a killer when it came to keyboard mapping- it was one of those rare songs where I never played a sound from the keyboard I was playing- it was always from something else because of what had to go where...
IIRC, I re-mapped the Korg M1 drums to have the sounds on the lowest octave so it could be played on my MKB-300, and I played it with my left hand whilst the right did the choir accompaniment under the melody. I think I mapped the sounds to C, D, Eb, F, because the chord for the verse was Cm.
You're right: it's not very difficult- it is one of the easier ELP pieces, on par with "From the Beginning," I'd say
..Joe