In ny humble opinion the B18C Integra98 engine, which goes into the Civic '93, is the most interesting one.
Not a crazy amount of power indeed, but enough to have fun with the Civic.
Also the Civic '93 I'd consider it a nice and friendly plataform to step into the tuning area. I've been doing that myself.
* Seeing this thread sometimes makes want to start a new GT7 game.
Edit: the K24A, if not mistaken goes into the Honda Touring Car it's another interesting option. Swapp the engines and set up the top speed in the gear ratios and the car is mostly done, because it is a well balanced car already in its original form.
Edit 2: be aware that engine swapping is a no return process. Once you swapp an engine you can't revert the process and install the original unit back in your car. Sure, there's an option in the tuning section "new engine" but somehow it doesn't work with swapps. The same goes for widebody changes. Once you install it, it can not be undone. If you choose the option "new body" it will revert the weight reduction (if you did any) and the body ridigity. But the widebody stays. PD game designs....
I'd advise you to have an extra copy of the car you want to experiment the swapp then. If you like it, you keep it. If you don't, just sell it and GT7 life goes on with your extra car.