The contribution might not be much more in itself, but its value to the Allies could be much greater without Japan even becoming a belligerent. The UK would be delighted by a deal with Japan in summer 1940 to trade DEI oil and Malayan rubber for escorts and merchantmen as Dathi suggests.
Unfortunately, the trade interests were peripheral for British and US in 1940. IOTL Britsh preferred to burn their oil (search for "Petroleum Warfare Department") instead of seeking a trade partners. Therefore, i should agree Dathi Thorfinnsson, with some important fixes:
1) don't let junior officers make public policy by assassination - already done in 1936-1939 period IOTL
2) don't go into China (beyond Manchuria, anyway)
3) realize the disparity between the US and anyone else (particularly Japan) in terms of economic and industrial power - Japan leadership did know. They just did not find better options.
4) build lot and lots of tankers and destroyers and merchant ships. When WWII breaks out, sell them to the Allies for oil and other strategic resources. And a greater say in China. - Already done in 1943 IOTL
5) Formally join the fight perhaps late in the war, to be at the negotiating table.
Therefore, the critical POD for Japan going for Allies would be (2) - avoidance of the Marco Polo Incident in 1937, and avoidance of the full-scale war with China thereof. In this case (5) - a late entrance of Japan into war on the Allied side - may be plausible. British were already highly critical of the Kuomintang government human rights policies, therefore China may be diplomatically isolated even if not joining Axis outright.