Yes, it's fine to shoot the (new to you) A303 without the cutoff button installed, as the gun will operate...the only thing you give up is the ability to utilize the magazine cut-off, which in my experience is not needed.
Take the gun out and enjoy shooting it.
If the lack of the cut-off bothers you, then contact MidwayUSA, Numrich or Midwest gunworks and order the needed parts and install them.
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