Romania is a great country to visit. It has tonne of cultural sights, a lot of interesting history. And the roads are generally in good condition, however, riding backroad adventures, you will get the occasional terrible roads with big potholes. So take it easy.
Yes. Although Romania is part of EU, it is not Schengen, so they will check your identity at point of entry.
Green insurance card: No
Technically not needed, being an EU member country. And they will not ask for it. But get it anyway.
Vehicle Registration Certificate: Yes
Yes, always. The original.