Though the Ki-45 wasn't useful as an escort fighter, I do believe that it mirrored the Bf 110 in being a good platform for night fighting (though it could have benefitted from a radar program comparable to most other major combatants), fighter bomber and maritime attack against smaller vessels. I don't see any other Japanese type - operational or not - besting it here in the historical time-frame.
Japanese, or at least their Army air service, were not in the need for night fighters until late 1944? Lack of the radar certainly makes any Japanese NF severely under-performing in it's task. As a fighter bomber - looks like there were no bombs or rockets carried ever? Just one cannon (neither of the types installed in the nose were great) for two engines seems like a waste for me*. Other people - including Japanese - used just one engine on a fighter that carried 2 cannons in 1941-43 (or 1, or 4 cannons in Europe). A smaller vessel (from a 1500-2000 t destroyer down?) was probably more afraid of 6 gun HMG battery on a P-40 than it will be afraid of a Ki-45.
* late in the war, it carried another pair of cannons in a 'schreage musik' layout
tl;dr: I'd disagree that Ki-45 was
that good in anything. Hence this thread
It may be possible (and even advisable) for Mitsubishi to produce more of a better engine, but as is, I can't imagine a better use for the three and a half thousand engines that powered the actual Ki-45's. Any single engine fighter using these engines would be mediocre at best, and we'd need 1700 more pilots.
Without the need for 3400 Zuiseis, Mitsubishi can make more Kinseis and/or Kaseis. Granted, not at 1:1 ratio, but probably 3000 more Kinseis? A Kinsei can probably fit on the Ki-43 (= 350-360 mph?), it certainly can fit on the Ki-61 or Ki-44, and it might be possible to fit it on the Ki-60.
A Kinsei-powered Ki-61 leads to the type being much more reliable in service, and more of them means less of pilot wastage vs. the pilots flying a Ki-43. Similar with Ki-44+Kinsei vs. Ki-43. Less wastage = more pilots available, and pilots with greater experience to boot.
A nod to using the Ki-60 as the base is that it was a small aircraft, wing area was comparable to the Ki-44. It might even work with Zuisei, that had about the same power as the Sakae in 1941-44; although I'd try to slap the Kinsei on it.
In a what if everything can of course be tweaked, but without serious deviations from what was, I do find the Toroy to be about the best that could be achieved.
It might be interesting if Kawasaki went to a small, 1-seat 2-engined fighter. Talk wing size as on the Whirlwind (250 sq ft wing instead of a 340 sq ft), with two cannons (imperfect as the were) + two HMGs in the nose? Not having to fiddle with burried radiators leaves a lot of space for fuel tanks in the wings. A drop tank facility that can also accept bombs?