Hello I'm new in this forum, but I wanted to share my latest effort, hoping somebody will refine what I did or will give a different take ;)
Following the discussion about the Suzuki Katana, I too was not satisfied with two aspects of the model: it had the shifting mechanism on the wrong side of the bike, and it included a V4 engine, when the original bike features an inline-4.
For the shifting mechanism:
I managed easily to mirror the steps of the instructions and ended up with the pedal on the correct side. It required me to modify a couple of things in the paneling, but very few extra pieces were needed, close to none. The only real problem is with the kickstand, which touches the newly added pieces. I didn't want to spend too much time on this mod, so I ended up filing a bit of the end of the kickstand, since it is a fairly common piece, but I feel like there's definitely room for improvements. Maybe with more extra pieces the kickstand as a whole can be changed to end up with a better solution. In the end the paneling needed close to no modification, only moving the technic-panel on the bottom left. Both the brick built panel as well as the technic built panel required no rework.
Regarding the engine:
in this pic is not so clear, but I managed to fit a mini-scale engine, using the freed space, and connected it to the gearbox through a couple of gears and rubber bands. I had to slightly modify how a couple of bodywork panels are attached to the bike, but the position didn't change. Not fully happy with this solution, but I feel it's an improvement over the original model, being much more faithful to the original bike.