Assuming we still experience some form of Sino-Japanese war but Japan does not conquer China, perhaps only mucking about in Manchuria and the few other vassal states it was tinkering with, the warlord era might last a bit longer and China might be smaller, and if this China needs to re-take Manchuria and defeat Japan then China might be as badly damaged.
From some roughly late-1940s to present day I think China has at least the same population and achieves a similar economy.
Industrialization may occur a few decades sooner or might be starved of capital longer. What I think would boost the economy more is the proliferation of small business. I think China can get self-sufficient and grow its internal economy faster, something that may not boost the wealth but could improve quality of life. I think we tend to look for more and bigger smokestacks, the Chinese need basic industry, a supply of shoes, farm equipment, rugged transportation, phones, the building blocks, they have labor, ambition and skill galore. If anything I would project a better launch when China enters the global market. And not merely as the place you do the dirtiest industry and use masses of cheap modern Coolies. Overall I think this China could transition at some earlier date if the great body of Chinese entrepreneurs are given an earlier start and left free of Communist doctrine squelching them, it might look rather Hong Kong like in its rapidity.
It is this shift towards urbanization and industrialization that will slow population growth. And how does the family see a daughter? Does inheritance and family lineage remain traditional and centered on the son? The sooner that is altered the quicker the trend towards one child or even no child can occur. And avoid famines or spasms of killings and one sees more than just an uptick, the stability can promote a tapering of population growth.
The KMT will be authoritarian and democracy will be long in coming but it might occur bottom up faster, and I think in the village level domain, in the towns and cities that might prove more beneficial. China has a history of top-down rule that may see little change, but I think the impulse to cooperate might make a rather effective democracy despite how some titular head rules. To me any Communist Party stays authoritarian past its prime, this China may see a nearly similar strong handed rule but I think the potential of the KMT is to shift into less authoritarianism faster.
Despite the real awful aspects to the KMT, I am optimistic for any ROC and the Chinese, it will have its own atrocities, failures and short comings, but I think it has every chance of giving China a less rocky road back to her genuine place as a great nation.