If it is like my 150, thinking it is, it is easy to pull the PTO shaft. 1st is to drain the oil, two plugs. Then four bolts around the shaft holding the flange. Put a bolt thru the hole in the shaft (splines) for something to get a hold of. Some shafts just pull out, others need some effort. I had to wrap a small chain around shaft/bolt, attach some weight to chain a gave it a swing reward.
Yes, replace the o-ring, I think there should be a one sided sealed bearing in there. Been a while and memory does fade :-)
Would also be a good time to pull the left side round cover, while the oil is drained, to check the pump intake strainer.