First, determine the desired bore size. Find a hole saw thats as close as possible in OD to the cylinder skirt of your new kit.
Then, find a hole saw that has an ID thats the same as the ID of your cases. Use that hole saw to drill out a plug that will fit snugly into the cases to support the pilot bit of the hole saw. Use the hardest wood or plastic you can get to help the pilot hole last longer, this will help everything get started more smoothly.
Pop your plug into the cases. Then, take your hole saw that's the OD of your skirts and get drilling. You can do this with a hand drill, but a drill press would make things more foolproof.
Use high quality hole saws if you can, they will run much more true and will help things go smoothly.