Find the same model HDD somewhere, (type serial number into ebay etc) get same model, take it apart carefully and change the arms?

Will be a delicate job as you will need to pull the platters out too, but at least if you practice on the one you buy you can be prepared for the one you are repairing.

If it's a really old HDD, obviously will be harder to get a spare unfortunately.

Or, if the arms are too hard, change the PCB and platters over into the one which is working.

I've got about 6 broken drives here if you want more practice ...