Sorry for the delay. Just returned from vacation.
I've never used a radio to find ignition noise frequency. The frequency changes with engine speed so it's a moving target.
The cracked coil won't cause any extra noise. The epoxy doesn't shield EMI or RFI at all. When we get coils like that in the shop, we inject epoxy into the cracks to keep dirt and moisture out to prevent a possible short to ground.
Normally, I'd recommend 16 ga. wire for such connections. If you're using some kind of multi-conductor shielded cable, then 18 ga. will work fine for your Vertex.
Spud