The ratio will be stamped on either the left or the right lower lip of the differential case not the cover.

According to the 1986 TDM 201.122 with a 5 speed manual transmission came with a 717.410 (GL 68/20 A-5) tranny and the 3.23 differential up to October 1985 and a 717.411 (GL 68/20 B-5) and the 3.91 differential starting in November 1985.

As far as potential problems, you should be mindful of the connections between the tranny and drive shaft and the drive shaft and the differential. The tri-lobe yokes come in a couple of different hole diameters with the basic rule being lower HP smaller yokes.

As far as alternative differentials you'll probably need to stick with 124 chassis parts, the rear covers might be swapable as far as mounting the diff, but another concern to be mindful of regarding differentials there are two different axle shaft/connecting flange sizes 86 mm and 94 mm where the majority of 124 cars use the larger, as do the 87 2.5 Turbo cars and the 190E 2.6 cars where most of the other 201 cars use the 86 mm axle and flange.