The S uses regular 7" units. This is the size used on lots of British cars including the old Mini and Landrovers. Suppliers like Stafford Vehicle Components can supply new units with H4 halogen bulbs or try your local motor supply shop. Avoid the sealed beam units.
The outer chrome trim is basically the same as fitted to the original Mini but with a L shaped bracket riveted where the screw hole is. This bracket can be made from a small strip of alloy or stainless steel.
Alloy strip riveted to outer chrome bezel.
The inner chrome ring that attaches the headlight to the bowl is supplied with the bowl. The model to use is the Wipac S5400 headlamp mounting set.
Good light units are no good if they are not getting the full 12 volts. It is worth checking the connector plugs behind the front inner wheel arch to ensure that they are not corroded and are in good condition. Check all earth leads as well and if you have a multimeter then check the resistance from the negative battery terminal to the earth side of the bulb. Disconnect the positive terminal of the battery first though.