ROKA Air Force - pre-war:
As illustrated by the two links below, there seem to the "official" line (see the 1st link) and unofficial lines (2nd link, for example).
The latter state that Rhee had about 1,900 men, including over 100 trained pilots (about twice as many as the official line) as of June 2, 1950. Rhee's AF had an odd mix of planes - Japanese planes left on Korean airfields in 1945, American observation planes and trainers from the US, and a squadron of fighter-bombers (F51D's?).
South Korean sources claim that the fighter-bombers were bought via a 'private' channel with money donated by the citizens because Amercica refused to give Rhee any 'modern' war planes. However, some American sources claim that Gen. Chenault recommended 25 F-51s for Rhee's air force. Gen. MacArthur rejected this recommendation at first but then 'reluctantly' approved it on Rhee's "I gotta have them' insistance, and the transfer began in 1949.
The official line is that the transfer began after the war started and Col. Hess (Rock Hudson) was there to prove it.
1) http://www.wpafb.af.mil/museum/history/korea/no71-2.htm United States Air Force Operations in the Korean Conflict 25 June - 1 November 1950