What relevance does a written agreement have?
If you borrowed it rather than rented it, it was a favour rather than a business transaction, and as such you have a moral / gentleman's agreement responsibility to return it to condition ASAP, and ASAP doesn't mean when conveinient to you.
He's out of order to insist that both wings are replaced, unless when the first is replaced they don't match. He shouldn't insist that they are replaced speculatively. That said, it's not difficult to see why his nose might have been put out of joint by the way you're acting.
From his perspective, it's not hard to conclude that he did you a favour, and that now you're finding every excuse under the sun (overdraft, stuff, working away, being paid late months ago, a mate who's fallen on hard times, 'finding out' you've been paid, etc) to be difficult and it's a slap in the face.
None of this is his problem. Between you and your mate, the moment you were back in the UK (if not before tbh) you should have been on the phone with a credit card to get things fixed, and gone out of your way to provide regular updates on what is going on, so he didn't have to chase you. It would then be your responsibility to figure out how to pay for it after it's fixed, and he's enjoy the rest of the summer, safe in the knowledge that he hasn't has the **** taken.