i had the same issue with mine. 5/8 is the same as 16mm to a tee. i never got an original tool kit with my bike. what i did was took a cheapo socket and used a flat file to take the walls down a bit. i think i took it down 1 mm maybe two and kept trying to fit it in the hole. have an extension going into the socket and shave it down especially the bottom till it starts fitting. be able to twist it with ur hands and dont use a ratchet to try and squeeze it down there. keep shaving and keep hand twisting and trying the socket. it will go thru the valve cover and into the deep well.
i went to all kinds of places trying to find it and relized i could make my own instead of getting a "specialty" tool thgat would more than likely be over priced. just thin the exterior walls and smooth out the path. if i had a pic of mine id post it. hope this helps.