1. I lost my watch. I must buy a new one.
2.I lost my watch. I must buy a new it.
3. I saw a big one.
4. He has a small one.
Is #1 grammatical? What about #2? Is #2 ungrmmatical? Can we use the pattern, 'a adjective one' only?
1 is fine; 2 is definitely not fine!!
Yes, your word pattern is correct, and even sometimes without the adjective.