One option would be to create a trust and have the car titled in the name of the trust. Your mother could then use the car for life but it would not become part of her estate. This would also protect you from liability due to her driving. Otherwise, your mother should have a will drafted that clearly indicates the car is to go to you on her death.
The lien will probably get you into Medicaid trouble, but if you make the trust the TOD owner of the vehicle, you'll still avoid probate and secure your interest in the vehicle.
Just a thought--I would compare the cost of setting-up the trust vs. keeping the title in your name and just having good insurance coverage + umbrella---and kudos for buying your mom a Porsche:-)
Legal Disclaimer: Richard W. Beck is licensed to practice law in Colorado. His answers are for general information and no Answer or Comment shall be deemed to create an attorney-client relationship or create any right of confidentiality. The reader should never assume that this information applies to his or her specific situation or constitutes legal advice. Therefore, please consult an appropriate attorney in your jurisdiction and who is familiar with your specific facts and all of the circumstances as there is likely a time limit related to the question that could expire at any time and you would lose any rights you had.