On Thursday, April 28, 2016 at 3:59:32 AM UTC-7, Ignoramus31353 wrote:
I sold a customer this hoist, and, unfortunately, upon delivery the
sight glass was broken.

He says that it is a round glass with a little groove for a o-ring.

He's probably wrong. Why would anyone bother to grind a groove in glass?
Cut or grind a glass disk (or find a crafts-store and look at the little round
mirror selection), use a bit of cork gasket, and you're good to go.