Hydraulic Steering Repair?

[jeff6645]

Having some issues with my steering and need some advice.

See the picture below I am missing the nut and what ever else is suppose to go on the bolt that is sticking up. Does anyone know what size of nut fits it? If the threads are damaged can the bolt be easily replaced?

I also need to rebuild the cylinder; is it very hard to do? The steering wheel has a Seastar label on it.

The motor is a Yamaha 250 v6 - I think it's 1989 model.

sorry for the oversize picture...