You can buy just the enclosure which will include the usb board.
All you need to know is the size of the hard drive 2.5" or 3.5".
If its a fairly recent drive it will be sata if its very old it may be IDE but you can still get IDE enclosures.
However if you buy another complete external drive then yes it will be a straight swap.
This is an example of a 2.5"enclosure.
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