are we talking a morris/ hunt magneto ?
Check the coil visually for signs of overheating
primary resistance will be so low as to not be detectable with a home meter ... so you're looking for continuity .
from wire to core . ( 0.3 ohms )
Secondary resistance between the two HT spring mounts will be somewhere around 13,000 ohms
the coil is held in place with two set screws into the core .
the screws may be internal or external depending on what casting was used .
new coils are as cheap as a 80 us dollars ... but more likely $110