I am a physical therapist for 20 years with experience in treating Parkinsonian patients as well as a flyfisherman and tyer.
First thing is consider larger instruments for tying. The smaller instruments make it harder to hold and control with the tremor. Additionally, the weight will help. Parkinson's typical is predominated with a resting tremor, which reduces with use. As apposed to a attention tremor which worsens with use.
The suggestion of a 2 handed rod is a good one. Less fatigue(which can be a factor with the meds).
Consider the environment you are fishing in. Climbing on rocks may not be a great idea as balance can become an issue.
The best plan is to stay fit. Don't let the diagnosis run you. A good exercise routine that includes strength, endurance, balance and flexibility is not just the recipe for longevity in fishing, but living with the disease.
Stay in close contact with the neurologist to control the medication and consider seeing a PT or hand OT prophylactically.
Stay well.
Tom