Not a good position to be in – you don’t want to be seen to be souring working relationships but only contact is through the workplace.
I’d get a formal written quote for what needs doing from a bike shop (he can’t argue) present it to him (with witnesses) and say you want the money within 4 weeks (or the next time you get paid).
Talk to your boss and explain what you’ve done and that you understand that it’s delicate but you thought you were doing someone a favour and don’t want to be out of pocket due to the way he behaved on the day.
IF he’s not paid when you said then I’d talk to your boss and say you have no choice but to go through small claims unless the company feels it can get involved due to it being a company organised event and they may be able to influence his behaviour.