Yes the second design is just right! I did a search for "cowells lathe rating plate" and found them on eBay this morning. I guess I'm not the first to wash the ink off the original, and someone has found a market.

The Cowells uses a pair of sewing machine belts, so really easy to buy and cheap as chips. The part number is MB-410, which seems to mean “machine belt, 410 mm”. If only all part numbers were so simple!

When I started the motor, the machine duly flung drops of oil right down my shirt. As if to tell me, "all done!"

This exercise began when the motor kept on stalling. I looked at the the motor pulley and saw the belt was running on the boss of the pulley, not in the groove. The last person who changed the belts fitted the wrong length, and used the larger diameter of the boss to make the bottom belt fit. So the ratio of the gearing was higher than it should be, and everything was running faster than I thought it was.

I wonder, is this the original motor or a replacement? If it is the original, surely it hasn’t got much life left?
I have read about people putting a 3-phase motor into these machines to give continuously-variable control of the speed of the motor. So fewer belt moves. I need to sit and wait, see how the machine behaves itself. I haven’t persuaded it to stall yet, and it is clearly running at its proper speeds. The new speed plate will finish the job when it arrives.