Is it right to pray to a priest or bishop who has passed away long ago?
Yes, we can pray to those who are not officially declared saints. After all, one of the criteria to be named a saint is that someone’s prayer was answered based on that individual’s intercession in heaven. If we could only pray to officially declared saints, then criteria for sainthood could never happen. And, of course, the Church enjoins the prayers of all saints (declared and undeclared) on the feast of All Saints.
The Church, however, only officially sanctions and encourages prayers to declared saints to avoid problems that might happen in venerating someone whose life was quite less than holy.