In a 1v1 my money is on the Whirly. From what I have read about both planes it seams that the Ki-45 was no match for P-40s in a dogfight but the whirly went head to head with 109 and 190s and managed to hold their own.
For attack roll the Ki45 would take that as the weapon load out is impressive, though having a 37mm gun on the nose would degrade performence somewhat.
The whirlwind is a very clean body and if it holds its E like a mossie does, this could be a very deadly fighter in the right hands.
The Ki-45's 37mm was fitted to attack american bombers, however the early models had a hand loaded 37mm that was horrible inaccurate let alone could only fire 2 rounds per minute. Later on I know the 37mm became automatic, I am not sure if the accuracy was improved or not or if it was even the same cannon.
One thing I always pondered why its gun package was never really upgraded, at least 2x20mm and 2x12.7mm would of at least given it a pounch.
Early models only had a single 20mm and pair of 12.7's, I believe the hand loaded 37mm came after that.
Basically its like firing the 75mm on the B25H, with less accuracy and a 37mm instead of a 75mm.