The first thing that comes to mind is Chiang and friends die in the Chinese Civil War before they can escape to Taiwan. Rather than give the island to the PRC, the US opts to return Taiwan to Japan - perhaps on the condition that Japan has to accept a number of Chinese refugees.
Likewise, have the US opt to not involve itself in the Korean war, but decide to hand over Jeju to Japan when South Korea is taken over by communists.
Japan + Jeju + Taiwan gets you to 152 million people.
No South Korea means more east asian investment goes to Japan too.
Increase Japan's population by ~20% and increase GDP per capita to 45,000 in 2017 (OTL it is 38,440 - in contrast to Canada with 45,000) then you have a country with a GDP that's probably around 1.4 or 1.5 times what it is today. That might seem like a lot, but New Zealand (who started way behind Japan) has a GDP per capita of 42,000.
Japan really needs to focus on agricultural reform (the agriculture lobby is strong) and not maintaining a full-employment model too. Japan has an incredibly low unemployment rate because it prioritizes employment levels over growth - resulting in companies often keeping companies that they shouldn't. If Japan were willing to accept a higher rate of unemployment they'd grow faster.
Could Japan be a conduit for finished goods to get to the USSR and around US trade restrictions? Let's say finished goods banned from export to the USSR are sent to Japan and then Japan marks them up and sells them to the Soviets via Vladivostok. That'd put a wedge between the US and Japan AND boost the Japanese economy. Then following the end of the Cold War lots of Russians, familiar with the wealth of Japan, move to Japan.
A good way to force a split might be to have Japan have strong economic ties with unsavory regimes. If the Japanese are trading with Than Shwe's Burma, Apartheid South Africa, Islamist Iran, Saddam's Iraq, etc in defiance of US pressure on these countries following the end of the Cold War, that could lead to a divide. Maybe the elites of these countries opt to send their students to Japanese universities and you end up with sizable expat communities in Tokyo strengthening trade ties and boosting population. T
One factor not paid attention to with regards to Japanese strength is just how much the company offshores its investment and manufacturing. Because the country is so anti-immigration Japanese companies with labor shortages often just set up factories in other countries. If you counted the involvement of Japanese companies overseas as part of the GDP of Japan, it'd be much larger.