OK, I've finally obtained that NTSC back-up of Crazy Taxi and as I said it didn't boot as a regular PAL back-up. I've patched it with the crazy taxi patch in this site and now it boots. The problem is that not only it doesn't run at 60 Hz, but it also has a very big black strip in the bottom of the screen (oddly enough there's no strip in the top).
I've burnt it again using the NTSC2PAL tool and obtained exactly the same result.

Is it normal? Is there a way to play the game in more favorable conditions.

Now that I'm here, I'll also comment that I've tried the opposite: I've taken a PAL back-up (Silpheed), patched it to NTSC with NTSC2PAL and it boots. I think it actually does at 60 Hz, but I get the screen displaced to the bottom, that is I have a black strip on top and miss the bottom of the screen.

Some method to fix that problem?

Thanks,

Dani