I thought messanic jews were christians who had an ethic background of being jewish as I have known some in my life and they claim this.
They are Jews that accept the Messiah, (Christ/Jesus) they do not cease to be Jewish because the coming of the Messiah completed OT, thus they are "Complete Jews" or Messianic Jews.
Since Gentiles can't be Jews (or we would all technically be Messianic Jews), gentiles were required to call themselves something else, so we were called, "Followers of the Christ" IE: Christians.
Now to get technical, a Christian only needs to follow the NT as that was the only covenant that God made with the Gentiles, where a Messianic Jew would follow the NT and OT.
In today's age, Gentiles study the OT to understand the history of the Christian religion and this is where Messianic Jews would guide us, as it is their living and study of the OT that they can share with Christians to help us build our faith and show us the relationship between the old and the new, so we can appreciate the deep rich history of the relationship between God and Man.
It is the Messianic Jew that is supposed to convey this understanding, because they are the
only ones that can (and have) accepted both the new and old covenant with God. Christians, as devout as we might be can't accept the Old Covenant, as it was never offered to us.
I hope this helps you out, confusing, but it quite simple really.
God Bless.