If you have to put on a new rotor, replacing them in pairs is probably a good idea, so the friction characteristics are the same. If you have a good used one laying around, you can probably get away with changing just the one. As long as the replacement caliper is the same as the original, and the other one is working properly, you should be OK

If you end up drilling out the bolt, if you are careful and/or lucky and get the hole started dead center, increase the size of the bit gradually until you start to see the threads. Then you can peel out the last of the old bolt as a spiral. Not easy to do, but VERY satisfying when it works.

Anti-seize compound is another possibility to use on the threads when going back together.