The question is how much of China's population growth decrease can actually be attributed to the one child policy.
Pretty much every country in Asia has had declines in fertility. Heck, every country in the world has experienced it to some degree - places where women used to have 9 kids now have 5 for example. Even the poorer asian countries like Cambodia and Laos have fertility rates below 3 children per woman. India's is below three.
China's GDP per capita measured in purchasing power (rather than nominal GDP) is around 18k. Thailand's is 19.5K. China's fertility rate is 1.6 vs Thailand's 1.5.
The fertility rate was already going down when the OCP was introduced. Maybe that bump we see after the introduction would have been higher without it.
The Federal Reserve Bank of St Louis attributes two main reasons: 70s economic growth and there already being family planning available (and encouraged) before the OCP. Would the KMT be able to provide family planning? I have no idea.
How wealthy would an RoC China be? Without the communists cleaning house, I can't see China having the rapid growth rates experienced from Deng Xiaoping onwards. I also doubt that the RoC in the mainland would be a copy-and-paste economic history of Taiwan. But, there'd be an advantage in that Mainland China from 1949-1976 is going to have higher growth rates than it historically did. Compound interest is a powerful thing, and given how the global trend in the 20th century more-or-less across nations was for more liberalizing reforms in the late 20th century, I can see China becoming friendlier to business over time and attracting more investment. I think Japan in particular would start setting up factories in China (awkwardly, I guess) as labor markets tighten in the country in the late-20th century. Anyways, TLDR I can imagine China ending up comparably wealthy (or a bit wealthier) as it is today despite lower growth rates if only because its growth period would be 27 years longer. Mexico today has a 20k GDP per capita measured in purchasing power, so I guess that could sort of be a good analogue. Comparable growth rates would mean a similar demographic transition, which would mean China's population won't be drastically different from OTL.
However, another big difference for a KMT china would be no cultural revolution or great leap forward. If you're not killing off the professional class, then that likely means some improvement in the economy as a whole (unless the professional class was holding the whole system back to protect its class interest, which I doubt was that likely in a totalitarian communist state).
18 to 56 million people died in the Great Leap Forward. Let's say the number is 40 million, just by averaging out the various figures wikipedia lists off and rounding up to a multiple of 10.
In 2011 the Chinese Government said that the OCP probably prevented 400million births, but then adjusted that to say that the OCP + preexisting policies (family planning, etc) resulted in 400million fewer births. Other demographers say the OCP probably resulted in 200million fewer births, which I could sort of believe.
Also, 6 million people died in the 1945-1949 portion of the Chinese Civil War. The KMT also stripped the mainland of much of its wealth to both fuel the war effort and to bolster themselves on Taiwan. If the KMT opts to consolidate in China proper and not go after Mao in Manchuria, more or less avoiding the 1945-1949 fighting in China proper, then you have another 6 million people alive from 1949 onwards.
I guess a KMT China would have 300 to 400 million more people. Although you'd also need to subtract Mao's Soviet-backed Manchuria (~122) and East Turkestan (~8) and add Taiwan (~24). So that nets you 194 to 294 million more people.
I guess there's the question of what happens to Hong Kong and Macau here though.