You could just run with 5mm bullets, just have to be a little more cautious making sure the polarity is correct since you wont have a housing to make sure that the polarity is correct.
Using a 120A HW ESC and a Castle 3800kv (5mm shaft) on 3S with Mod1 gearing I was able to hit 77MPH with my Rusty before I took it to 1/8 scale electronics and 6S.
One of my favorite setups for my Rustlers is a sidewinder and a 3800kv combo. Of course 4 is a little better than 3 and Id pick the copperhead over those but any of them will work.