Where does the New Testament say that Jewish (Hebrew) Christians may continue to observe the Torah?
When the temple was destroyed in AD 70 that was the end of Torah observance. But even before that, the day Christ died on the cross and the veil in the temple was torn from top to bottom, that was the end of true Torah observance. God sent a clear message to Israel -- the temple is finished, the Levitical priesthood is finished, the temple sacrifices are finished, the feasts, festivals, holy days, new moons, and sabbaths are ALL FINISHED. The Law of Moses is finished. The shed blood of Christ has brought in the New Covenant, therefore under no circumstances may you return to the sacrifices of the Old Covenant.
So on what grounds can any Christian (let alone unbelieving Jews) claim that believing Jews may be Torah observant? That would be telling God that they will defy Him and Christ. And it would also be a fake Torah observance, and God hates hypocrisy.
The Torah requires the physical circumcision of Jews, and if Gentiles were to be Torah observant, they would have to follow this practices (or be exposed as playing games). But what did Paul say about this matter?
But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God. (Romans 2:29).
Under the New Covenant, circumcision is spiritual -- the death of "the flesh" (the old nature). Therefore God has set aside the Old Covenant altogether. Any Jew who is converted must become A HEBREW CHRISTIAN, and that means he must put himself under the New Covenant.
Not only that, but under the New Covenant, God has abolished all distinctions between Jew and Gentile within the Body of Christ. So if a Jew continued to be PARTIALLY under the Old Covenant, he would be VIOLATING the New Covenant by maintaining the distinction between Jew and Gentile.