Putting anything on a gravestone is considered a no-no nowadays. The current recommendation is a bright light shone across the stone, e.g. wait for the sun to be in the right direction, reflect sunlight across with a large mirror or unrolled aluminium foil (having an assistant helps), or use a bright torch. The last preferably in the dark if you have access at dusk or dark, can avoid tripping over on the way, and won't start a ghost story

Torches with a row of LEDs can be bought in "pound" shops (though inflation means they may be a bit more than £1).
However, enhancing the contrast of Find a Grave's expanded image looks to me like 18
58 is the correct date.