John,
The ball and spring acts as a breather non return valve that creates a partial vacuum in the crankcase. This assists in drawing oil down from the drip feed sight glass. The valve vents the crankcase pressure (when the pistons near the bottom of the stroke) into the timing chest. This gas contains oil mist that keeps the valve gear lubricated. In the bottom of the timing chest there should be a short standpipe to control the oil level. There should be another pipe on the outside of the timing chest that routes any discharge so that it falls on the road clear of the back tyre!
The ball and spring used to be a stock item in LDMCC spares.
Regards,
Eddie.