The problem is that the law enforcement authorities could easily contend that the simple fact of a police car on the road projects the power and authority of law enforcement, and if a car was built and driven by someone who is not an officer, that would constitute impersonation because people would assume only a police officer would be driving a police car. The police could cite the driver for impersonation; the merits of the case could be decided in court, but it's speculation on what conclusion would be made.
There's always the fact that Oregon law could be quickly amended to outlaw personal or custom vehicles which simply appear to be law enforcement vehicles, if the legislature saw the need in response to police requests or requests from the public.
Current Oregon law can already interpreted to include those who impersonate the police in a vehicle. There are several recent news articles:
One from ktvl.com:
Anyone caught impersonating an officer is facing a Class C felony
charge, with potentially up to three years in prison.
And another: Police impersonator pulls over Oregon driver, turns himself in - oregonlive.com:
...turned himself in to police on Friday evening, OSP said in a
release. He was cited and released for criminal impersonation of a
public servant and disorderly conduct.