Is my marriage valid legally

Even now you both go to Marriage Registrar's Office and get your marriage registered and you have to ask about the procedure for the registration of marriage at the office concerned.

He is not ready to register the marriage. But our marriage was performed according to hindu rites in the presence of 50 people

Convience your husband for registration of marriage so that you can travel / stay with him as his "wife"

The marriage having been solemnised in accordance with the custom of the community, the husband cannot shirk the responsibility in getting in registered. You may approach the Family Court and seek appropriate directions to him.

Is it mandatory that a hindu marriage between an Indian woman and Overseas resident of India should be registered 

Where was your marriage solemnized, in India or abroad?  You say that your husband is/was an American citizen? It appears that you both are normally living in India. In India your marriage may be valid. But in USA or in any other country, it may not be vaild. Actually you are in a difficult situation. Consult a lawyer with knowledge of marriage and family laws.