Karnak please provide the reference you are quoting in saying the P-63 and P-39 didn't share a single part.
The P-63 is on display in the wright Patterson airforce museum in Dayton Ohio. So is the P-39. The Ki-43 is not. Please list the American Museum where a Ki-43 is on display. I think American planes are more commonly known than Japanese planes to casual observers, or even hobbiests, with the exception of the Zero. My father was a kid during wwII and he knows all about the P-39 and P-63. He has no idea what Ki-43 is.
To continue to debate the popularity of the Ki-43, and how much of an upgrade the P-63 was from a P-39 as a defense for why the P-63 is not int he game is...silly.
Of course they shared some parts. They had the same guns for Christ's sake. The structure was entirely different though, you could not take the spar from a P-63 and use it in a P-39, nor the ribs, nor the fuselage formers, ect, ect. I really don't care if it is added eventually, but right now I see huge gaps in the Japanese, Russian, Italian, British, German and French planesets while the US set is very well covered and people have descended into asking for US aircraft that practically didn't see service.
And my grandmother is still 100% sure the Japanese-American workers at sugarcane plantations on Oahu cut arrows to direct the incoming Japanese strike to Pearl Harbor despite the US Navy investigating those claims during WWII and concluding they were false. Russian and Japanese aviation were almost unknown outside of their respective nations, even to the USAAF, USN and RAF. The fact that a civilian had never heard of a given aircraft type is meaningless. As somebody who has put a goodly amount of effort into the study of this subject, but focused on the war and not just pulling aircraft from books listing types from the era, I knew what a Ki-43 was long, long before I ever heard of a P-63. In fact I did not come across the P-63 until I looked in the books that just list the basic stats for many different aircraft from the period. Studying WWII aircraft from the standpoint of studying WWII you will come across almost no mention of the P-63, while the Ki-43 fought from 1941 through 1945 in all places the IJA fought.
EDIT: Japanese aircraft were given other names by the US because we often had no idea what they were called by the Japanese, and even if we did the names were hard for English speakers to remember. The Ki-43 was the Hayabusa to the Japanese, which means Peregrin, but to US pilots it was the Oscar. Any WWII book where the author talks about fighting against Oscars, those are Ki-43s.