Just been through romania and wasnt aware of any special speed limit in force for bikes, although you are right there are very few bikes there in comparison with the other eastern european countries i passed through. Ý would suspect if there is such a law it relates to the size of the bike or age of the rider but regardless throughout Romania i saw many police officers, and they saw me, and regardless of my speed showed me no ýnterest at all, i was in romania for about a week.
A carnet is not requýred for Romania and as for insurance i was not covered on my green card section of my policy, be worth checking yours, but was not required to purchase any thýrd party insurance at the border eýther.
Hope this is of use, if you need anything else let me know, otherwise have a great trýp in what is an amazing country.
Some day so soon....