Orwell actually called the book Nineteen Eighty-Four, but even that was transformed into nineteen eighty-four on the cover of the first edition — and the figure 1984 appeared as well.
There have been many editions, some featuring the title spelled out in words, others using the figures. The 1987 Penguin edition I have, first published by that house in 1954, spells out the title in words and features the original copyright attribution prominently, "Copyright 1949 by Eric Blair. All rights reserved." Given that, it's unlikely that his estate would have given permission for Penguin not to use the original title in its original form, and it should be spelled out in full.
"1984first" by George Orwell; published by Secker and Warburg (London) - Brown University Library. Image via Wikipedia