I think the original lenses, being standard equipment, were never sold separately. You may only find one from a dead Start and Starts seldom die. Your other option is to get a Start bayonet to 39mm adapter (from fedka, of course:-), then you can used any 39mm SLR lens, including a usual Helios-44. There will be no auto aperture, but many 39mm SLR lenses have preset feature, which is as convenient.
Recent series of Helios 44 M5 M6 and M7 can competently be adapted to Start cameras One must construct the bayonet and external knob. It is not difficult since you have competent machinist. I can furnish drawings.