Buck: The 1839HQ series are definitely known to work and be reliable.
If a cap fails, it shouldn’t make your motor quit instantly but the uF rating will shoot off the high end of the scale and the points will burn prematurely causing a miss. Test their rating again after running the motor for a while, maybe 20 minutes, again after one hour, and see if it has changed any.
When I started experimenting with caps, I bought a bunch from Mouser but they had a lower voltage rating and might have had a different composition than the Vishay HQ caps and they wouldn’t last. Now I have a bag of them collecting dust.
This tool has to be one of the best investments I’ve ever made as it identifies the item you are testing and gives an accurate reading in picofarads instead of microfarads. It will identify different types of transistors, diodes, resistors, and capacitors etc. I keep one in the house for working on vintage audio equipment and another one out in the garage as a dedicated outboard tool. Highly recommended!