The Polonaise was written first, the Andante spianato was added afterwards - I think on the suggestion of a friend - and was written just for solo piano.
The only orchestral parts usually included in solo performances are the fanfares and transitions. As Joe Wallace said, it's not that much although certainly the (string) fanfare that sets off the polonaise is a little tricky until you get the hang of it. Any other orchestral contributions - i.e., those made while the pianist is playing are limited to doubling cadences and the like and aren't normally incorporated into the piano part when performing it as a solo piece.
(If you haven't heard Horowitz' solo recording from the 1940s, it's well worth a listen although it may not be to everyone's taste. His virtuosity in this piece is explosive!)
all best,
jeff ok