I used the same coil as you. I successfully did two of these conversions. I didn’t have any trouble soldering them after I cleaned the terminals. Perhaps our techniques were different. I started with a small puddle on my iron to help transfer the heat. It took a minute or so until the terminal came up to temperature. Have you taken a continuity reading of the coil to see if the windings changed? I think mine came out around 6 ohms or so. I don’t remember the exact number. If you coil is truly shot then you could practice soldering on it until you find the best way to make it stick. Maybe starting with an electronics flux will be necessary. Sorry I can’t be more help.

Also, don’t try to tin the wire directly to the coil without first having tinned both parts separately. You will have much better luck this way. Trying to tin both at the same time may result in too much heat at the terminal since the added mass of the wire draws heat from the puddle. I tinned the terminal first and then moved on to tinning the wire to allow the terminal to cool down. Once both parts are tinned they will be easy to stick together.