BC81740MTF is an onsemi that has gone obsolete, however there are viable alternatives available.
The part that currently looks best positioned to use as a replacement is BC817-40-TP (DigiKey Number- BC817-40-TPMSCT-ND or BC817-40-TPMSTR-ND). It appears to be a good drop-in replacement.
Please review the datasheets to ensure the alternative will work for your application.