When you allowed your friend to borrow your motorcycle, you gave him constructive possession. Basically, you allowed him to use it in any reasonable manner. The only legal remedy to you since 1)there was no contract and 2) you allowed him to possess it at the time, is to sue him for negligence. You would have to prove that he had a DUTY to care for your motorcycle as a reasonably prudent person would, he BREACHED that duty, the dereliction of that duty was the CAUSATION of the theft, and, as a result, you have suffered DAMAGES. Causation is the major element here. You would have to show that his conduct (or lack thereof) made the theft foreseeable. As in, he left the keys in the ignition, he knowingly took it to a shady part of town where it was stolen, etc. It's all about proving that he didn't act as a reasonable person would in the same situation.
Since a civil suit is expensive and friends are priceless, I would put a lot of serious consideration in commencing a suit. I, too, would be very, very upset at my friend. However, you can always sue the slimy thief once he is caught. Why don't you do a little work of your own and post flyers, do some internet searching for your motorcycle in online ads, etc.? The quicker the thief is found, the quicker you can initiate a civil suit against him in addition to the grand larceny criminal charges he will face! Sure he might not be wealthy, but you have a few years minimum (depends on the state) to attach a lien to garnish wages, etc. once you have received a judgment against him. The best part is that you will salvage a friendship.

My opinion is that your buddy has to reimburse you. You were a good enough friend to lend it, its on him to replace it.

I was able to find this as an example case of a lawsuit over property stolen in another's possession.

And, this forum is a good resource for this type of legal advice.

Despite the fact that he did have your permission to borrow your motorcycle still it is but proper that your friend should reimburse you to replace what you have lost. He is civilly liable of it and thus he should pay you for whatever damage he has caused. Well, if you would want to know more about motorcycles or decided to purchase another one once your friend reimburses you with some amount of money good enough to buy a new motorcycle then don't hesitate to go over our official motorcycle website and from then on you can get amazing articles and watch videos to help you decide buying the best motorcycle for you. You can start by clicking the source link below. Thanks ahead and God bless.