Well Sam, do you feel as if you have been taken advantage of? If this guy was a real friend he would put his hand in his pocket and say "what do I owe you".
I would add up what has been spent, tell him your labour was free, and ask for your cut when the bike is sold.
Real friends like your so called mate don't treat people like the way he has treated you. If he can afford to do up his Cobra he sure as sh*t can afford to pay you for your efforts. I wouldn't be thinking twice about it.
You know, there are givers and takers in this world and I think you have clearly defined where both of you stand regarding that.