What are the various attributes or qualities of Lord Narayana as per the scriptures, for eg. omniscience, omnipotence, etc.?
Lord Vishnu has infinite attributes. He is called "Aprameya" meaning who is undefinable. He is beyond the reach of shastras, our senses to define hm completely. Hence, we cannot get a complete idea on what are his attributes completely. We can define him and his attributes based on what is said in the texts, our experiences but that would be equal to a drop in the ocean.
However, Vishnu Sahasranama stotram present in the Anushasana Parva, Mahabharata gives thousand names of Lord Vishnu which are indicative of his attributes.
yāni nāmāni gauṇāni vikhyātāni mahātmanaḥ|
ṛṣibhiḥ parigītāni tāni vakṣyāmi bhūtayē ||
I am giving for the benefit of all, those names of His which are far-famed, which are indicative of His attributes, and which find a place in the exalted hymns of the ṛṣis.
You can have a look at the names and attributes given in the Vishnu Sahasranamam with a commentary of Adi Shankaracharya on shivkumar.org.
Parthasarathy, K.E. adds Adi Shankaracharya and Sri Parashara Bhattar's commentaries in his book Sri Visnu Sahasranamam.
There are many names and attributes which are not listed in the Vishnu Sahasranama because the ones named in the stotram are popular ones. So, the number of attributes are limitless.
True, Vishnu is limitless as confirmed by these verses in Srimad Bhagavatam. In Bhagavatam 1.18.3, we find
O ṛṣis, who are as powerfully pure as the sun, I shall try to describe to you the transcendental pastimes of Viṣṇu as far as my knowledge is concerned. As the birds fly in the sky as far as their capacity allows, so do the learned devotees describe the Lord as far as their realization allows.
In Bhagavatam 11.26.30, Krishna says
What more remains to be accomplished for the perfect devotee after achieving devotional service unto Me, the Supreme Absolute Truth, whose qualities are innumerable and who am the embodiment of all ecstatic experience?