Using the same action with minimal alterations I would say 6,5x64 Brenneke (read 6,5-06), brass can be made from 270 win cases.

Going for another 30 like a magnum will be costly since you need to alter bolt face, feed lips, new magazine/ follower and so. You can use the existing magazine unit but it will not be in correct size according to "Mausers design" Also make sure to have the actions hardness checked if you want a magnum of some soft.

7x64 Brenneke is also a nice cartridge.