How to calculate cutoff frequency for this filter?
Increasing C6 will reduce the frequency of that stopband zero from 45.4MHz to 41.1MHz.

Reducing C11 will tend to widen the passband and upset the passband shape.

I suggest you get a copy of LTSpice, a free, fully featured simulator, enter the circuit, and see whether you like the result. This will allow you to play with the results of other component swaps.

A hint. Changing the values of odd components in an existing design rarely give you a nice filter, especially when zeroes (Cauer filter) are involved. The flatness of the passband, the depth of the stopband, tend to require all of the components to be at their design ratios.