Searching works. "DR350 manual" will lead you to:
The Great Greg Bender has a manual on his thisoldtractor site. Download it and include him in your bedtime prayers.
Gas cap should be keyed to your ignition. If you'd like to remainlremain lockable you can buy a cap and pay to have it rekeyed. You can buy a cap and ignition keyed alike. You can buy a cap and key and add a key to your ring. You can buy a non-locking cap too. Your call. Ebay is an easy place to find all this. Others sources are a google away.