Hi Dax;
Your wheel-bolt pattern is known as "190pcd" (NOT 198---the mk2 126 had the standard Fiat pattern of 98pcd). There are a number of suppliers on the European continent who can supply front disc-brake conversion kits for these cars (standard 500). If you DO fit front discs (discs are NOT required on the rear), I would suggest that you go up a width or two in wheel size,which will allow the fitment of wider tyres. If you retain the 3.5in wide (original) wheels with disc brakes fitted you will find it VERY easy to 'lock-up' the brakes, especially in the wet.
There are a number of wider alloy wheels available that are 4.5in wide, or do you have a specialist in the area who can 'band' steel wheels? By widening the steel wheels you will be able to retain the original wheel trims (if your wheels have them fitted) and will look fairly original.