As a slight aside, it might be kind of neat if the ride was able to obtain some sort of traditional kind of Japanese scroll (weatherproofed I guess) signed by representatives of Honda Japan as a token of support, good luck, goodwill, may the good spirits bless your endeavor, etc.
Back on topic(ish) I don't think there's anything wrong with a piston. It tends the be the generic kind starting point for a lot of different cycle and automotive logotypes. I even have one tattooed on my arm. I'm thinking though that it might be too generic for this application. Something more directly tied to the sohc4 factor might be appropriate:
I know this is specific to only one of the motors, I'm just offering it as an example.