I haven't used one of the Astroflight motors myself, but you can buy them from the factory
www.astroflight.com or many other places online if you do a bit of searching. From what I understand, the astro cobalt motors are pricey (upwards of $100) but worth it. Many people now are skipping over cobalt motors in this size range for brushless motors, they are more efficient and powerful, but for the most part far more expensive and require a brushless speed controller. IMHO, the ferrite 600 with the 3.5:1 gearbox and the 10X5 prop you ordered
should make a significant improvement. BTW, that seems pretty decent of Tower to offer to fix the plane. Good luck with this setup. Maybe someone else will chime in with their opinion, but I say you're definitely on the right track!