You're not going to like this at all at all:
Unlike desktops, motherboards for laptops are almost all custom-per-model, so the only real source is the laptop manufacturer (either the brand or the actual maker, and there's often a restriction preventing the actual manufacturer from selling parts outside of the brand channel). It's a form-factor thing -- the connectors have to be where the holes in the case want them to be, cutouts for the battery or drives have to be in the right place, and so forth. (There was a "standard architecture" movement for a little while among the Asian fabs, but it made for bulkier machines than people were willing to buy.) So that means that you need the actual 5W610 motherboard for your 1100 (although you might be able to Frankenstein the case to make the MB for the Inspiron 5100 fit -- there's an S-video jack that needs a new hole).
I couldn't find a source for a new MB for you; just
some refurbs at around $150-175 (and a "used" one at around $90 that looked pretty sketchy). Oh, and remember that if you want to try the 5100 Frankenstein upgrade, you'll need to get a video card as well (it's not integrated like on the 1100).
Replacing the MoBo in a loptop is quite the chore. You pretty much have to disassemble and reassemble the whole computer, and the plastic case parts are generally uncooperative. If you've never done it before, schedule a full work day and make sure you follow
the instructions to the letter -- otherwise you
will break things.
Once you've spent the money and the time, you may or may not find that you have a working laptop. The 1100 was one of those models that had the "suddenly burst into flames" battery problem (you may remember hearing about the huge multi-brand recall a few years back), so there may have been thermal damage to any of a number of components other than the MoBo that you don't know about. The battery on a laptop of that vintage is very probably shot, even if it was replaced during the recall period, so you might find that you have no more than a few minutes of portable time. The memory is DDR (not DDR2 or DDR3) SODIMM, and that's expensive if you need to replace it, and the hard drive is IDE (Ultra ATA/PATA), so the choices are fairly limited (and more expensive per GB than the current SATA drives). And at the end of it all, you have an eight-year-old entry-level computer that can't be upgraded much (you should be able to build it out to 2GB RAM and I think you can find a 250GB 2.5" PATA drive, though 40 and 160 GB drives are a lot easier to find).
In other words, unless you're doing this for the experience, it's not really worth trying to fix. If you had a bunch of them, it might be worthwhile to make one or two good ones by swapping parts around, but if you have to buy parts it gets really expensive really fast. You'll have to invest nearly half the price of a modern (minimally-specced -- but those min specs are what the 1100 would be maxed out) laptop, and there's no guarantee at the end of the day.
Sorry to be a downer. If it were anything but the MoBo, I'd say go for it -- at least you know that what you're replacing is all that needs to be replaced. If it's any consolation, the SODIMMs, keyboard and display are probably worth something to a parts/repair guy (and can be used with more than just that Dell), so there may be some profit in it in any case.
