Modelling a low-pass filter on LTSpice to filter an input square wave at 50kHz to obtain a sinusoidal output at 50Hz
What you're asking for can't be done.

A linear filter like the one you drew with a sinusoidal input always gives an output at the same frequency as the input. It can attenuate the signal differently depending on the frequency, but it does not change the frequency.

To produce a 50 Hz output from a 50 kHz reference, you need a frequency divider or a phase-locked loop circuit, which are substantially more complicated than your RLC filter, and also require active elements of some kind.

Edit

In comments you wrote,


  from my understanding, a square wave is made up of many sinusoidal harmonics - attenuating the higher order harmonics will not result in a sinusoidal 50Hz waveform?


The harmonics of a square wave are at odd multiples of the fundamental. So a 50 kHz square wave has harmonics at 150 kHz, 250 kHz, 350 kHz, etc. If the duty cycle is not exactly 50%, there will also be components at the even harmonics: 100 kHz, 200 kHz, ... There is no frequency content below the fundamental.

You could conceivably start with a 50 Hz square wave and filter out the harmonic at 49.95 kHz or 50.05 kHz (Of course picking out just one of these would require a very high order filter). But you can't start with 50 kHz and select out a harmonic at 50 Hz, because 50 Hz is not a harmonic of 50 kHz.