You say that
メーラルアン村に不時着した日本軍機1 seems to assume that a single engine aircraft was involved. The sketch on that webpage reinforces your point.
I had my Thai translator check Cherdchay's interview in Thai in 2008 for mention of number of engines --- there was none.
And she called our current 83-year-old witness to ask about the number of engines --- he said it had been too long ago and did not recall.
I believe that nouns in both Thai and Japanese languages, standing alone, do not give information as to number, ie, single or plural (eg, in English, engine vs engine
s). Hence, since there is no specific mention of number of engines, it could be logical to assume that, if the plane that crash landed at Mae La Luang had two-engines, which would have been "out of the ordinary", that detail would have been worthy of mention; but it wasn't, so, in fact, the aircraft was single engined.
21st Sentai: Wikipedia
Kawasaki Ki-45, lists 13 IJAAF "operators", and one of them is No. 21 Hikō Sentai IJAAF; none of the other units with Ki-45s were in the Burma Theatre (except, curiously the 71st Dokuritsu). And, as you opined, Wikipedia also lists the 21st Sentai as having
Nakajima Ki-27s. But Wikipedia does not list units using
Nakajima Ki-43 Hayabusas.
Okay, I see that Shores (vol 3, p427) had the 71st Independent Flying Chutai using Ki-51s; and that chutai was absorbed by the 83rd Sentai in late 1943.
Umemoto lists no IJAAF loss in 1943 for unit number 71 or 83. I can't check for a Ki-51 casualty because he doesn't identify Japanese aircraft using "Ki" numbers.
There is one serious obstacle to speculating that it was not Ushijima who crash landed in Mae La Luang: Umemoto offers two key details that, in combination, point very strongly towards Ushijima and his Ki-45:
- no loss of life
- a riverbank
Umemoto's books came out in 2002; Cherdchay's interviews were conducted several years later, in 2008 --- and after, and were eventually translated into Japanese and put on the Internet. It's possible that the Japanese translator or the webmaster was familiar with Umemoto's listings, checked for an event that did not involve death but did involve a river bank, an unusual detail, decided that Ushijima's crash fit those details, and added a photo of a Ki-45 to the Internet article. Whether the guess was right or wrong, lucky or misleading, is not clear.