Hello LordRain,
Replacing the Ha 140 engine with a Kinsei would NOT be an easy task.
As Tomo Pauk pointed out, there were some other changes, but those were not that bad as compared to the need to completely remove the two side walls of the forward fuselage ahead of the firewall which are the original engine mounts and fabricate new tubular engine mounts.
To me, this seems like a task for a factory and not one for the typical wartime repair shop.
That isn't to say that it is impossible. After all, if they can raise the Yamato and boost it into space, this is pretty trivial.
Since this is a bit imaginary anyway, how about pulling a German Daimler Benz 605 from an aircraft the Germans may have sent over to Japan for testing?
- Ivan.
View attachment 614909