These are vague... are you available to train a replacement anytime, even after your last day? Are you saying you'll be too busy to help prepare someone else or that you'll be busy helping with the transition? As with everything else in your resignation letter, clear and concise are the way to go.
After that, you're ready to close out your letter and move on. The basic closings are fine here__again, you don't need to go overly sentimental or formal, but you should also write as if it will be read by anyone at the company. And with that signing flourish, you're done! You've resigned, and you're ready for your exciting new job.