Mannlicher Schönauer magazines are specially caliber-fitted and re-fitting for another caliber is not so easy work to do. IMO Mannlicher Schönauer m1903 with other caliber than 6,5x54 MS isn't M1903 anymore.

By the way; here is for sale quite nice mounts to Mannlicher Schönauer:
http://www.egun.de/market/item.php?id=5486303