Hi Vonm and welcome to IIS , great scope you have there and the Parks mirror cell is an oldie but a goodie from yesteryear, very good gear.
On the SV eyepiece,s the 20mm is very good , I have one and I have done a lot of testing of this against my beautiful TV 19mm Panoptic and it holds up very well , close, almost identical out to 70-75% of the field where the TV is better , perfect to the edge even in my f5.5 sky90 , but in your longer Newtonion it won't be to bad. I would suggest Plossl,s but. for only a little more the SV,s are the ones.
Leave the mirror alone for now. , just collumate it. And enjoy , its a great scope you have.
Brian.