Unless the seller cooperates, a Paypal dispute will require that you return the entire item to the seller with Confirmation of Delivery in order to get your entire original payment refunded.
You would be responsible for the return postage.
Give the seller until Monday (or two business days, which ever is sooner) to respond. Ask him if he will accept a picture of the damaged part as proof of the problem. Ask him to suggest a remedy.
If he is being difficult, go for the PP dispute.
Are you looking for a partial refund to pay for the broken part? Are there even replacement parts available? I've never seen a plane that didn't have some replacable parts. But mine belonged to my father (died 1975) and grandfather (died 1948) so more modern ones may not need separate blades.