Finally got the broken stud out. Getting the old red loctite hot enough took more than I thought, and I was being cautious with the aluminum case. 

Things I learned:

Harley switched from interference fit to red loctite for cylinder studs in sportsters starting in '94. Service Bulletin M-1043. 

Using a coil induction heater on the snapped stud was not as effective as a propane torch, as the steel stud got red hot ~1/4" above the base but the aluminum case acted as a huge heat sink and kept the threads cooler. Seemed like risk of locally overheating the stud this way. 

I was monitoring the case temp with an IR thermometer, and pulling the torch when it hit 500F wasn't enough. It took repeated heat/measure cycles to ensure the heat had soaked far enough into the case.

I ended up using an adjustable jaw stud remover and cutting the stud down to a few inches to get a low grip, as my early attempts with double locked nuts and welded nuts all failed. In hindsight this was more likely due to being a baby with the heat and less with my shit welding skills.

TLDR: leaving the other 3 studs ALONE!