Find all positive integers $n$ such that there exist non-constant polynomials with integer coefficients $f_1(x),...,f_n(x)$ (not necessarily distinct) and $g(x)$ such that $$1 + \prod_{k=1}^{n}\left(f^2_k(x)-1\right)=(x^2+2013)^2g^2(x)$$
To solve the problem, we need to find all positive integers \( n \) such that there exist non-constant polynomials with integer coefficients \( f_1(x), \ldots, f_n(x) \) and \( g(x) \) satisfying the equation:
\[ 1 + \prod_{k=1}^{n}\left(f_k^2(x) - 1\right) = (x^2 + 2013)^2 g^2(x). \]

1. **Analyzing the given equation:**
   \[ 1 + \prod_{k=1}^{n}\left(f_k^2(x) - 1\right) = (x^2 + 2013)^2 g^2(x). \]
   Notice that \( f_k^2(x) - 1 \) can be factored as:
   \[ f_k^2(x) - 1 = (f_k(x) - 1)(f_k(x) + 1). \]
   Therefore, the product becomes:
   \[ \prod_{k=1}^{n}\left(f_k^2(x) - 1\right) = \prod_{k=1}^{n}(f_k(x) - 1)(f_k(x) + 1). \]

2. **Considering the degree of polynomials:**
   The left-hand side of the equation is a polynomial of degree \( 2 \sum_{k=1}^{n} \deg(f_k) \). The right-hand side is a polynomial of degree \( 4 + 2 \deg(g) \).

3. **Matching degrees:**
   For the degrees to match, we must have:
   \[ 2 \sum_{k=1}^{n} \deg(f_k) = 4 + 2 \deg(g). \]
   Simplifying, we get:
   \[ \sum_{k=1}^{n} \deg(f_k) = 2 + \deg(g). \]

4. **Considering the constant term:**
   The constant term on the left-hand side is 1. For the right-hand side, the constant term of \((x^2 + 2013)^2\) is \(2013^2\), and the constant term of \(g^2(x)\) is the square of the constant term of \(g(x)\). Therefore, the constant term of the right-hand side is:
   \[ 2013^2 \cdot (\text{constant term of } g(x))^2. \]
   For the equation to hold, the constant term of \(g(x)\) must be \( \pm 1 \).

5. **Considering the structure of \(f_k(x)\):**
   Since \(f_k(x)\) are non-constant polynomials with integer coefficients, \(f_k(x)\) must be such that \(f_k^2(x) - 1\) is a product of factors of the form \((x^2 + 2013)\). This implies that \(f_k(x)\) must be of the form \( \pm (x^2 + 2013)^{m_k} \) for some integer \(m_k\).

6. **Finding \(n\):**
   Since each \(f_k(x)\) contributes a factor of \((x^2 + 2013)\) to the product, and we need the product to be \((x^2 + 2013)^{2n}\), we must have:
   \[ n = 2. \]

Therefore, the only positive integer \( n \) that satisfies the given equation is \( n = 2 \).

The final answer is \( \boxed{2} \).