I've run both my conversions with standard nitro shells with a big hole. Allows for plenty of air flow and easy temp monitoring without having to take the shell off. If you have to take the shell off to check the temp, then you're not getting a true operating temperature. Fastrax make a good gun. 120F on the motor is OK, you can safely get hotter than that. I don't get too stressed until about late 60's C.
PS I have to hole-less shells and won't use either of them
You might be Ok with that combo if it's all you can afford. The Blitz is only 2WD and it's a very light car even though it's big. I used the same combo in a Madrat which had similar sized wheels and weight to a Blitz and it ran very well. HPI use the same 9T in the Bullet Flux which is 4WD with big wheels. The 8.5T / 60A combo would be better. The cheaper combo will run hotter and probably have a shorter life, but you'll get away with lower spec batteries, so it's much cheaper all round. The 60A esc also has the advantage that you can run it on 3S which makes the Blitz very quick.
Daveyboi on here used to make metal replacement spurs, may be worth contacting him, he may still have some. If not, I have an unsued one if you want, but they're not cheap,
Pretty sure mine is just on stock gearing, using the same EZRun 60A / 8.5T combo you have. There's lot's of info on this site about gearing a Blitz - http://www.rcshortcourse.com/forum/hpi-blitz/
I'm selling one of these http://www.modelsport.co.uk/fusion-exceed-ultra-550-monster-brushless-combo-7.5t/rc-car-products/34542 if you're interested. Probably want about
The motor is turning backwards. Either swap two of the motor leads around or, if you have the option in the program menu, you can reverse the motor direction. That should fix it.