Certainly on both my 2.0 spiders (1978) the bearing fits inside the tensioner pulley, it's retained by a circlip. So if your's is the same read on.
The spare bearing I've got is a 5205-2RS. The 2-RS bit means 2 rubber seals, the 05 is the bore size in 5mm steps (ie it's 5x5mm=25mm bore), the 2 means (IIRC) medium duty and the first 5 means it's a double row angular contact bearing. The odd thing is that it's a mixed units bearing - 25mm bore 52mm O/D and 13/16" wide (20.6mm)
The good news is that any bearing factor will be able to sell you a 5205-2RS. Ask for a decent make such as SKF, RHP or FAG.
Stuart - who used to sell bearings in a previous life !