the mdl file is 100% compatible with fs2004. 

the an225.air is modified a copy of the default boeing747 so you 
can use the fs2002 version of that file. fs9 modifications in
the .air file affect only the center-of-gravity cause ive noticed
that fs8 aircrafts lift off with their main gear first when 
used in fs9 and that the entries in the aircraft.cfg are not 
noticed by fs9.

the aircraft.cfg was an 747-400 one too, but was modified
to get the an225 heavy feeling and dynamics. you can open 
the 747 aircraft.cfg into any editor and copy/paste the
modified lines of the fs9 an225 aircraft.cfg into the 
fs2004 version. 