The SF-20-300 is a Williams coil and is a little stronger than the 21-50008B Gameplan coil.
300 vs 375 turns respectively. Lower turns = stronger.
You could try a GamePlan 21-5002B with a 400/1000 (second number is the hold strength)
Reference this site for more info:
http://www.flippers.com/coil-resistance.html