I believe the new run of xtras from graupner now have an enclosed cowl(no precut muff. hole) which I would have preferred. And no special servo arms are recommended, I used original 'adjustable' arms fully extended. The original engine mount works perfect for the .91fx. But to clear the landing gear your going to want to get the muffler extension (tower hobbies part# LXARP9 @$16.99),This will give you the clearance needed and looks good too!
Good luck and happy flying!!