I did a bunch of runs between Astoria and Seaside a few years ago. Used the Lewis and Clark Rd. and hit the mainline before Crown Camp which was nice. I did it with both an electric setup and a 4 stroke gasser.
For sure don't be running your motor on the riverfront path please as that will just make folks mad. My 4 stroke bike pedals well enough without the motor going with a tailwind that I used to take it on there sometimes and I did it both ways with my electric because it was silent but kept my speed down around others.
Both my rigs could take 8th st and get to the Column with ease btw.
The big thing with OR motor law is that they restrict motor assist bicycles to 35cc or less and 25 mph. That pretty much makes China Girls illegal in OR. In fact not many options outside of friction drive or Golden Motor types. My 4 stroke is a 49cc and never got hassled with it but played it very cool around town for sure but wasn't afraid to open it up on the back roads.