The 148 series machines were manufactured by Soryu. I only have one 148 machine (or perhaps two), but I believe the width regulating lever is spring-loaded with the missing knob being a limiter. Is that correct?
Kenmore knobs can be found on eBay from those who've parted out their machines; such knobs can be an expensive solution (these sellers appear to charge as much as they can/dare), although if the machine is important to you, it would be the best route. Since your machine was only $20 (possibly less than a replacement vintage knob), you might do better to wait and buy another Kenmore machine for a similar price or less. I've been able to find CL Kenmores for free. One of my favorite Kenmores was about $6 from a thrift store, in great shape.