You should look for an identical driver to the original. Otherwise, it will not be right. The crossover was designed for the original driver, not for anything else.
No, it won't be OK. It will cause your midrange to disappear. You might be able to make up the difference with an active bimaplification system but that would cost more than simply getting the right drivers in the first place.
sure you can replace it and it will make sound-but nothing like the original.
It depends on whether this is important to you-or you just want to make noise
Actually, it is possible that he will need to give them the serial number as well, because sometimes manufacturers make changes in productions without changing the model number. But they will tell him that if it matters in his case.