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.
State Your Intent. The whole theme of your resignation letter is the statement that, well, you resign. You don't need to play coy__the time has passed for you to fish around for a counteroffer if it's gotten this far. At this point, you've made your decision to leave, and this letter needs to reflect that.