cranks but no spark at coil

Something like a #8 gauge insulated butt connector should work. It is not easy to guess gauge in photo. Heat Shrink Butt Connector Red 8 Gauge - Bt-8hs - Heat Shrink Butt Connectors - Butt Connectors
If you need #6 Ace 6 AWG Insulated Butt Connector - Wire Clips & Connectors - Ace Hardware
You will also need decent large crimp tool.
I would cut back the crap soldered ends, clean copper with clean 3m green pad, crimp lower 2 in one end, the upper 3 in other. Slip additional heat sink first, use it as secondary insulation and seal.