The easy way to repair the instrumentation on the superb and very popular classic CB750 machine this speedometer suits is to renew the complete assembly.
Alternatives:
Order new fasteners and anti-vibe cushion to secure this assembly and perhaps a new speedo cable.
CMS your one-stop online classic and modern Japanese bike parts shop. Competitive prices and shipping costs combined with knowledgeable and enthusiastic staff to help you get exactly the parts you require.