You may be aware that the IJN in 1939 bought the He 100 D design plus production jigs for license production. Three He 100 D-0 arrived in 1940, but neither plans nor jigs were delivered to Japan. If you scratch the Jack, replace it by the He 100 which might carry the IJN designation of J2Hei1 and become available in 8/41 (HA40 engine production provided, which might be the major availability driver for the type). For details see the WIKIPEDIA entry for "He 100". That aircraft is a clear candidate for any "what if" mod.
One aspect of the original Projekt 1035 was the intent to capture the absolute speed record for Heinkel and Germany. Both Messerschmitt and Heinkel vied for this record before the war. Messerschmitt ultimately won that battle with the first prototype of the Me-209, but the He 100 briefly held the record when Heinkel test pilot Hans Dieterle flew the eighth prototype to 746.606 km/h (
463.919 mph) on 30 March 1939. The third and eighth prototypes were specially modified for speed with unique outer wing panels of reduced span. The third prototype crashed during testing. The record flight was made using a special version of the DB 601 engine that offered 2,010 kW/2,700 hp) and had a service life of just 30 minutes. Prior to setting this absolute speed record over a short, measured course, Ernst Udet flew the second prototype to a 100 km (62 mi) closed course record of 634.32 km/h (
391.15 mph) on 5 June 1938. Udet's record was apparently set using a standard DB 601a engine.
However isnt the Hien the Japanese version of this ? " And The Ki-61 was the last of the DB-601-powered fighters and it was soon overshadowed by fighters with more powerful engines. By the time it first flew in December 1941 – only one year after the Macchi's first flight and three years after the Bf 109E – the DB-601 was already underpowered compared to the new 1,120 kW (1,500 hp) inline or 1,491 kW (2,000 hp) radial engines being developed (and already nearing the mass-production stage) to power the next generation of combat aircraft: the P-47, Fw 190 and Bf 109 G. Moreover, the inline Ha-40 engine proved to be an unreliable powerplant."