If it is not a sealed beam . Try spraying glass cleaner in and then wash it in the sink with fairy liquid. Rinse out thoroughly. Maybe with deionised ( battery/iron ) water so you don’t get spots. It’s only glass after all so it is.
This has been recommended by others on here7" sealed beamI keep meaning to invest in one for the Spada as that still has an ancient bulb and Cibie lens that the previous owner fitted back in 1979! Wire in some relays at the same time is the recommendation to boost your night vision.
I have one on my Cali’ 3, along with a high performance bulb rated at 120%, transformed night riding.
If I remember, the standard headlight lense is plastic, the Ring replacement is glass.