transpose a score from mutopia

[mj]

Hi,

I am trying to transpose a Messiah lilypond file from mutopia: 
http://www.mutopiaproject.org/ftp/HandelGF/HWV56/Messiah/Messiah-lys.zip

However I'm not getting very far, could use a bit of help....

Extracting the zip gives a folder structure like:


   |-common
   |-Haendel
   |---Oratorio
   |-----Messiah
   |-------AAsinfonia
   ..... (many more)
   |-------CGchorus
   |-------CHairSoprano
   |-------CIchorus
   |-------CJchorus
   |-lilypond-patches
   |-out
   |-templates

So, looking at this, there is a main.ly in ./Haendel/Oratorio/Messiah 
which I opened in frescobaldi, but I can't compile it, as all the paths 
in main.ly look like "\include "Haendel/Oratorio/Messiah/common.ily"

Am I supposed to copy that main.ly to the folder folder above 
"./Haendel", to make the paths work, or am i approaching this the wrong way?

(the ultimate goal here: printing off a transposed version of a 
particular aria)

I hope someone can give me some pointers here. No big hurry, as we still 
have some time left before christmas :-)

MJ