First up, the crime could be anything, so long as the would be perpetrator(s) are desperate enough to not be apprehended and can get their hands on some Kryptonite. It might have to be something newsworthy if it's out of town; that is, they'd have to be concerned that Superman might hear about it and respond, so it'd have to be big enough and public enough for a country-wide newsflash. And it'd have to be something that would or could attract attention, so a covert bank robbery, for instance, in which the perps would much prefer that it wasn't discovered until they were long gone, wouldn't be that much good. A kidnapping or hostage situation would seem to be the best bet, though not the only one by a long chalk.
As far as the PU goes... well, you can ring any changes on the universe that you like to strand your character. Maybe the machine isn't there in the new universe, but it will be in a week's time (the person who built it in our universe is still finishing it in the PU); or the act of moving from one universe to another can only take place at certain times, and the next "window" doesn't open for a week. There's lots of possibilities...
Hope this helps.
Phil
