The bearing on the sprocket end of the clutch shaft must be a self-aligning type. If I recall correctly SB originally used a double-row one and Rover changed to a single-row one, but I haven't heard of that causing a problem. Deejay will set me straight if my memory is wrong about both those points.
Do not attempt to use a deep groove ball bearing instead of a self-aligning bearing. The engine and sprocket will not line up well enough for this to work. Yes, given enough time you could shim up the four corners of the engine, and set it up with a dial indicator to precise alignment horizontally and vertically, then dowel it into place on the mower frame and all might be well with a deep groove ballrace at the sprocket end. However we both know you aren't going to do all that, so a self-aligning bearing is the only practical solution.
Long ago, veteran cars used to have their engines and gearboxes shimmed and doweled to the chassis. More recently they have been machined much more accurately in the factory to eliminate the shims, but the gearbox is still doweled to the engine's cylinder block. Lawnmowers are not built with car-like precision, and their frames are not especially rigid either. The result is to rely on self-aligning bearings, which will work well enough for their relatively short lives (measured by hours run, of course).
