The spool? As in the magazine? Takes more than just changing the mag. You'd have to re-chamber. Norma makes .303 Savage brass. Runs $27.99 per 20, Stateside. Finding it, here, is another question.
Epp's list loaded ammo. No mention of what brand or how much. Likely Norma.
The .300 Savage wouldn't be a great idea. It's a rimless cartridge with a smaller case head. The .303 Savage is rimmed. It'd require altering the bolt face and extractor. Neither would the .307 Win be a great place to start. It's case head diameter is also smaller and is semi-rimmed.