Hello Martin,
I had the same bad experience when loading to my new PSR SX900, a lot of midi files that sound very well in my old PSR2000 and that I need/appreciate. The alternative, nothing to do, you have to edit each file, changing in some cases to a newer voice, adjust EQ parameters like volume, reverb, chorus, sometimes modulation, etc etc, and now they sound real better than before, but believe me, It's a lot of work to do depending on the ammount of midi files you have, at the end you will be a pro i editing midi files. It's difficult to find on the web new versions of these old midi files, they were created few years ago, and still ramain that way without any new updates, so, you will have to edit your files, according to your taste and needs, beleive me.
Best regards
Bernardo