I reckon you can probably return it - distance selling law, if nothing else. Tell them that although it works, you didn't know you needed single density, and they should understand.
As for the motherboard; well... there must be better Pentium 3 motherboards out there. I don't think it lasted that long as a chip but it'd maybe be worth having a look around to see if you can get some cheapo second hand stuff that gives you the extra capability. On looking into this problem I also found you can only put up to a 32gb hard disk in your current machine.
Another problem is that it may not be a standard form factor; it might be some weirdo HP motherboard size or case design, but I have no idea about that.