Calvin,
On the OC-4 the seat and fuel tank have to come off. The pulley assembly comes off the rear, but I don't know about access from there. My shop manual is at the mechanic who is doing the work on the crawler so I can't tell you much. My Father owned an OC-3 but it was traded for the OC-4 in 1958. I was 8 years old at the time so I really don't remember much about the thing. What I do remember is that my Father got an impressive amount of work out of that OC-3 for the size of the thing. Sorry I could be of more help. Maybe someone else reading this has more experience with the OC-3's can be of more help. 
We have had a very mild winter so far. We did get about 4-6 inches of snow in that last lake effect weather pattern but today it is 44 degrees and the snow has settled to about 2-3 inches. We had just a dusting of snow last night, but the weather reports are calling for rain and snow mix over the next two or three days.