Hitech are great servos for replacements. That's all we use. Even the standard Hitechs are three times better. Airtronics are good, Futaba are good and traxxas are good. Only thing is you can't use a digital servo with the stock radio system. You need to use a non-digital servo, unless you upgrade your radio system to digital.

Servos compatable with Redcat stock system:
[link=http://allaboutfunhobbies.com/store/catalog/advanced_search_result.php?keywords=servo]Compatable servos[/link]