A competent General instead of Percival, and an Admiral instead of Phillips. Between April when the general arrives, and December when the war starts this general needs to train and exercise his troops vigorously up and down the Malayan peninsula. Operation Matador needs to be updated to reflect the forces at hand rather than those requested, and the lines of authority need to be clarified so it can be put into action in the confusing times at the start of a war.
The General then needs to conduct a force-on-force withdrawal campaign down the Malayan peninsula, using his greater troop numbers and being on the defensive to offset the japanese superiority in armour and in the air. This will cause the Japanese to exaust their limited supply stocks and they will have to stop for resupply somewhere on the peninsula to restock to sustain a further advance.
The Admiral should not steam srtaight to Vietnam on a rumour, although with a competent general he may not b put in this position. He should await the cruisers and destroyers which were mere days away from reinforcing him. He should protect the flanks of the Army and be ready to destroy the Smatra invasion force when it appears on Feb 14 1942.
Singapore survives for a time as does Sumatra, protecting the rear of Singapore. By late April Sommerville arrives in the Bay of Bengal with a fleet and the Australians are being bought back from Egypt. So round two begins.