This short review article (7 pages) was published in Sept. 2020, but as far as I can see has not been mentioned here before.
A couple of quotes.
"The focus of this paper is to provide a balanced review of the 'aerobic exercise and PD' literature, and provide rationale for the use of aerobic exercise as medicine for the treatment of PD."
"It is advised that PwPD exercise at an intensity of 60–80% of heart rate reserve or 70–85% of maximum heart rate. Given the prevalence of autonomic dysfunction in this population, 14–17 on a 20-point RPE scale is also appropriate. To put this into perspective, if one can exercise and hold a conversation, the intensity of the exercise should be increased."