Second part - you do not need to replace the rotor assuming its not worn out.
However you need to clean it properly. To do that I'd remove rotor from wheel, and clean every hole with degreaser or similar. Use an old tooth brush to get everywhere.
Dry it with paper towels, and absolutely refrain from touching it with bare hands or dirty gloves. Treat it like dentistry equipment.
actually boiling it in a pressurised autoclave might be a superlative way of cleaning - but I've not tried this. An ultrasonic bath cleaner might work too, again YMMV
While rotor is off the bike, give your hub and spokes a good cleaning too. There's very likely oil on the hub which could migrate out, ruining all your good cleaning work.
Once its clean and dry, refit to the bike. Use new bolts if the old ones are damaged at all, and remember some hand loctite on the threads.
As a corollory, find some way to stop this happening again. It might be as simple as two clean rags over the disks when you use any form of aerosol or spray.