I think you will not stand much of a chance regardless of having Lexus service the vehicle, once tuned/modified the car becomes your problem. Now you stated about him disabling methanol and he insisted not, personally you should have walked away with you car knowing the consequence. It will be Harding proving it is the company as you payed them for the runs and I'm sure they will have a policy in place that you are fully reliable for damages at your own risk. I wish you luck, if you can provide evidence and I mean strong evidence it is the company at fault then take it further, otherwise count your loss and get a cheap engine.