Nothing. There weren't enough of them. Five squadrons of Buffaloes, with a stark deficit of trained and experienced pilots operating from unprepared and poorly defended airstrips were tasked to defend a territory larger than the entire UK (defended by ~100 fighter squadrons in Dec 1941) against a much larger force of the IJAF's best aircraft and experienced aircrew.
Replace those Buffaloes with five squadrons of the latest Spitfire variant and it will make no difference. But give Malaya twenty or thirty squadrons of Buffaloes and (since there aren't enough Buffaloes) Mohawks and the Japanese will face at least credible opposition. Of course, even if the RAF fighter aircraft could be had, there's still the shortage of pilots, ground personnel and poorly sited and prepared airfields. The loss of Malaya was not the Buffaloes fault.