You want to convert RF to composite video and analog audio.
That's like trying to "convert" FM radio signal to an audio signal in order to feed an audio amplifier.
You can't get an $.89 adaptor to do this. This is not a matter of simply changing the physical dimensions of the plugs and jacks. They are totally different types of signals. You would need a whole tuner.
It's the same for what you're trying to do, except you now need a TV tuner as well.
The cheapest solution I can think of is a cheap VCR (used, new, mono, stereo.. .doesn't matter) and feed the game's RF output to the VCR's tuner input, tune the VCR to whatever channel the game outputs to and feed the VCR's video and audio outputs to your video center.
...you might wannna scour the local garage sales this weekend.