Make sure the motor isn't locked up. It won't hurt a thing to bump the starter for a second or two to make sure things spin freely.
Make sure the title / VIN numbers match.
Check for frame cracks. Shouldn't be any at 30mph unless he hit something solid.
Make sure the crack isn't in the engine case itself.
Pull a plug. Check for spark.
If all looks good and the price is right, go for it. Good luck on a potential project!
BTW - it needs to be cheap. You can get a very nice, and I mean VERY nice FZ1 of that vintage that needs nothing for $2500- $3500 all day long; many times with full luggage included. Make sure your project doesn't take you near that, or it wouldn't be a worthwhile project.