Jim,
Here's a place to get some info on Cosmics.
http://www.cosmic-wheels.com/page.php?4
As Ben said, they will be 5.5" wide at least. I have just found some 185/70-13 Falken Sincera tyres at a reasonable (?) price. Maybe not the best around but I do so little mileage hopefully they will outlast me.
Also have a spare MK1 Cosmic, but I think, Ben, you have MK2s?
Mine are as pic. I've been trying to get centre caps without success, but have found E36 plastic centres sorta kinda fit. I do have a feeling that the first bump I hit, all 4 centres will leave like a bullet from a gun!