Check this out. Here is my sprag assy., also the spring I chopped short to tighten its grip. Its shown in its linear form before I coiled it back onto itself, notice the amputated portion off to the side. I re-installed this spring and the starter works great.
The thickness of the OEM spring is 1.4 mm
Next to the sprag is the garter spring from a seal that was too thick; 2mm.
What you are looking for is a garter spring from an oil seal made for a 50 mm shaft of some sort, that is the critical diameter number. That's close to 2 inches for us non-metric folks.
What sort of things have 50 mm shafts?
-50mm white power fork dust shields.
somebody measure the garter thickness on these for us, I bet any brand of motorcycle fork seal in the 45-50mm diameter might work
-2 inch axle bearing seals for cars or trailers
I'm sure others can think of other possibilities and examples. I only bring this up for the poor guy who is stuck with a glassy-eyed parts dealer that says they can't get the magical SKF 19993. Armed with this knowledge and a vernier caliper you can start rummaging through seal boxes and find one that will work for your poor disabled bike.
Soon we will have multiple known options for even those across the pond and down under. The guessing will be over.