With the usual caveats (IANAL, etc), it would be fine. I think. So far as I know, most equipment-related regulations on cars follow the registration. Driving behavior, will be localized (e.g. double towing is still forbidden in Oregon even if you are titled and registered in California). I'm not aware of any Oregon regulation that says you can't drive a Kei car here, just that you won't be able to title & register it.