I've done this exercise bike powering stuff before - powered a TV to watch and a fan to keep cool. I got tired very quickly and couldn't watch tv anymore! I would suggest disconnecting the 3-phase bridge on the alternator and connecting a transformer to step-up the voltage, then feed that into the laptop charger. Although, you said that the laptop battery is nackered, you would be relying on this to hold the charge when you get tired. Alternatively, get someone else to pedal while you play on the laptop!