Paypal will refund you if they BMSB don't. The only problem is that Paypal might want you to send it back before they refund you, but there's a maximum that they will cover for the shipping cost, which is nowhere near what it actually costs. Things must have changed since Brexit because Paypal must now come under UK regulation with an office in UK. Before they were in Luxembourg, so impossible to go after. You're now fully covered by UK consumer protection laws as far as Paypal is concerned.
I'm not too sure that you can reopen the Paypal case once closed, so don't close it. You can live chat to Paypal or talk to them on the phone. they're normally very helpful and understanding, but they always insist that you stick to the rules and if you get one wrong, you're out.
What I'd do is write to BMSB and tell them that you will hold the motor and send it to anyone in the UK as long as they arrange the shipping, provided that they send you a replacement of the correct type immediately.