If they are leaving it to you  after they die, it will go through probate and the only documents that control what happens to assets after someone dies are wills. Anything you create that would say what happens to the motorcycle after the person's death is a will, and must be legal as a will (e.g. property executed and witnessed) and will be probated as a will.You could have them transfer the motorcycle to you now--either by selling it or as a gift--and in return let them lease it back from you (or license it to them) for, say "$1 and other good and valuable consideration, receipt of which is acknowledged"--that way, you'll own it now, so it will not go through probate, but they still get to use it. Because it will belong to you, they cannot legally sell it, transfer it, etc. to anyone else. The lease o r license agreement can state that they have to provide insurance (and what insurance you'd find acceptable) and that they are liable for any damage they do. You could come up with the contract of sale (if sold; assignment if gifted) yourself and lease/license agreement, but are advised to let an attorney help you.