You can replace the controller board all together, the electrical interface should not be that hard. You have to spin the spindle, jiggle the heads and send/receive the data. The main problem I see here is that modern hard drives have enormous, unimaginable information density. There must be so many tricks how they achieve that (and do that reliably!). You have to re-trace the steps the hard drive industry has made in the last 50 years.