It seems that the rhythm or ( strumming pattern ) is your main concern, considering the fact that you are playing the correct voicings of the chords. If you can not find the original score for this piece and check to see what the correct rhythm is, then you will have to decipher it by ear. This is a skill that many Guitarists do not have ( or have not developed fully ). It is very useful and studying Aural training will help you figure these things out more easily. Hopefully this post will help you explore Aural Training, as it is a very useful skill and will help you decipher the rhythm of this piece.

-Kyle Hicks ( Kole )
-http://www.myspace.com/kolemusic