All the parts are there, except for the inner seal. You're lucky to have retained the ball, which, like the interior seal, is "NLA". As discussed elsewhere on this forum, replacement balls (including cork) can be found on the Internet. As I recall, their sizes are expressed in millimeters.
When afloat—but not moving—outside water pressure floats the ball against the inner seal, which keeps most of the water from flooding backwards into the cockpit. The oval opening was designed to retain the ball*, so installation of the ball should be done from the cockpit end.
*Since the De Persia aluminum body is subject to production variables (and corrosion), oval openings can vary in size.
In the picture below, I managed to get the sun to shine down through the oval opening into the De Persia's nether regions. (You're looking down from the cockpit to view the interior). In particular, the sun has illuminated the groove that retains the seal. A regular garden hose washer can be made to fit, but it's not very supple. Attention to the chamfer can help to rectify this condition.
At .90, polypropylene's density says it should. But even if it didn't—as a solid polypropylene sphere—they are also made in a hollow form. (Both metric- and US-measure at link). I think you'd be safe with the solid ball.
I'm presently six hours away from my two De Persia bailers, so I'm unable to determine the proper size ball. The ball definitely can't be smaller than the opening vent—and needs to be slightly larger than the opening of whatever seal you install. To start, try testing with a ⅝-inch drill bit.
If you need more time to get the bailer to operate satisfactorily, the bailer hole can be temporarily sealed with a $5 expanding block seal of 1¼" size from NAPA stores.