I would second that it makes more sense to just replace the internal HDD of a 486 laptop with flash storage; you can use a 44-pin PATA to CF adapter (which is essentially a passive wire adapter), or you could go with laptop PATA to SD (the widget for that costs about $10), or possibly a PATA to M.2/mSATA adapter, although I've heard the last option there tends to be trouble with machines that have less than UDMA/33 controllers. If the issue is BIOS support for a larger drive then, and I know this is an unpopular thing to mention in some circles, drive overlay software like EZdrive can save your bacon without needing any hardware upgrades.
A few PC laptops can boot ATA PCMCIA flash/drive cards. If the Dell does then you could track down a PCMCIA hard drive, ATA flash drive, or a PCMCIA-CF adapter and go that route, but I think it's pretty unlikely you'll have the BIOS support for that.