1998? That's a
really early one! For the gearbox, it may be worth putting your VIN into the forum
ePER (it's on the blue bar at the top of the page). After selecting Language, Country and Left or Right Hand Drive, click through Spare parts > Fiat, then enter your VIN in the box and hit enter. You'll see a page confirming your vehicle's details. Click Next. From that point, any lists on the parts pages will grey-out anything that isn't suitable for your exact vehicle.
Go to the Transmission tab, then "21200 Transmission", which is the top-level page. Find the part number for the gearbox in your car in the list on the right (most parts for older Mutliplas begin with either a "46" or a "77") and make a note of it. If you click on the part number, you'll see a pop-up window giving details of the part and also a history log, where you'll see if it has been superseded by a different part number. Make a note of those part numbers as well.
You can then go on ebay and search for replacement gearboxes using those part numbers. Not everybody lists the part number in their item description on ebay, so it won't show you all suitable parts, but it's a good start. If you put the part number into google, you may get lucky and find links to other vehicles that the same gearbox is fitted to. I have a feeling that the 'box for the 1.6 was also fitted to some Bravos and Bravas, and possibly the Marea too.