Keynesian model functions
To get back to the original production function just multiply both sides with capital. Here:
$$Y/K = A (N/K)^{1.1} = F(N/K,1) \implies Y = AN^{1.1}K^{-0.1}  =F(N,K)$$
Also, the marginal product of labor here will be:
$$F_N'(N,K) =  1.1 A N^{0.1}K^{-0.1} = 1.1 A (N/K)^{0.1}$$