You need to find another SE FDHD or equivalent machine to snag parts from (probably a beat one with a dead screen).

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What Unkown K said. There was a kit sold by Apple, and occasionally such things show up on eBay (though rare), but it would be a shame to actually USE it to upgrade an SE at this point as it's collectable value would be substantial. The only way to do it practically and in any kind of timely manner, is to pull them from a dead SE FDHD logicboard or upgraded SE logicboard. Or just get yourself a working FDHD logicboard which turn up much more often. Likewise for pulling a 1.44MB drive, though reconditioned drives along with 400K & 800K capacities routinely turn up on eBay. This is true for almost all the "roll-your-own" upgrades. The kits that made it possible simply do not exist anymore.