Definitely do not have your horse go up to the motorcycle and have him start it all at once. That is known as flooding and it can be very traumatic, your best bet is most likely systematic desensitization. So as one poster noted, have the horse about 50 feet away, but I would not force the horse to stay. If he wants to leave calmly walk him away from the situation and repeat the process gradually getting closer until he no longer freaks out and he is right next to the motorcycle. Also, bring some treats for when he is being calm so he knows he is doing something good, as well as calm him down AND to associate the noise with something good! :)
I'm a psychology major so I'm glad I can use that money that goes into school for something practical!