In the movie of The Lovely Bones, what was the poem Ray wrote to Susie?

Apart from a very significant literary allusion, I am guessing that it was the writer of the screenplay for The Lovely Bones that wrote the poem you speak of (because Alice Sebold does not include a poem in the book itself).  Regardless, the answer to your question can be found in Scene/Chapter 5 at marker 49:05 - 49:27.

If I had but an hour of love,

If that be all that's given me.

An hour of love; upon this earth,

I would give my love to thee.

The Moore

It was specifically written for the film:  a nice addition, I think, to enthrall a teen audience.  HOWEVER, this poem contains a very important allusion (an indirect reference to a literary work).  In addition to Ray Singh speaking of himself as "the Moor" and referring to himself, then, as Shakespeare's Othello, there is also a line from Shakespeare's play echoed here in the poem:  "I have but an hour Of love, ... To spend with thee."  Ironically, the line is spoken by Desdemona and not Othello.  In this regard, the movie becomes a fun treasure hunt for any Shakespearean enthusiast.