Loving the Mini 4 Pro Bundle I bought but I have an old DJI FC40 that I killed a year ago when removing the safety guards by inserting the wrong size screws back into it.

I suspect its economically unviable to pay for 4 new motors even if I could still source them, but I have a chance of buying a set of used P330 brushless motors for £32 ($42), but need to know whether they're compatible.

Could someone please advise.

IIRC, the P330 was the original Phantom, later known as the Phantom 1.  I know the FC40 is an upgrade of the Phantom 1, but still considered part of the Phantom 1 series.  I would say there's a 90% chance they use the same motor.   Good luck.