My misinterpretation. I interpreted your desire to make the bike more driveable on the street as it wasn't running that well at lower RPMs. I also thought (because of reading things on the internet, which may or may not have been true) 270 cranks were smoother running than 360 cranks.
I think if you think changing out the crank is all that needs to be done, that is what should be done.
If the bike shakes disturbingly at lower RPMs after swapping out the crank, you'll know why I mentioned the head and fuel delivery system. The motor obviously wasn't built to be a street bike, and ridden at low speed. It's an interesting complicated racing design project.
If it runs great after swapping out the crank, I'm just another old fart on the interweb with my dumbassness showing.