I had the 5B last summer and the only thing i can tell you about motor, don't shaved any head or try to modify it, buy a hole new ESP engine. That's the best and you avoid lot of headhaches.
I've try to modify my engine, put head kit, reed valve, etc, and have lot of problem.
And the day i've decided to buy a complet Zen 26, ESP race ported, Wow!!!!!!!!!!!!!!!!!! no problems at all, and power to pass any of the best nitro truck.
This truck is a beast.
try it, and you'll fall in love with it