Take a cardboard template into your nearest glazing shop, proper shop not B&Q etc. ask them if they can cut a bit of thin mirror into this shape. If they can mastic it onto broken glass or actuation gear.
You could always check your local scrapyard for a cheap fix. The mirror glass is attached to a flat plastic area similar size to the glass, this is a complete unit and don't just try to attach a new mirror to this. The units come off their mount easier than they go back on but with a bit of patience it's not too bad.