Dont get caught up in the bigger cc better thing, What chassis are you fitting it to ???
I ran a tuned 23cc in a Lauterbacher supercross that would out run most things on the straight, Just my rubbish driving that let it down lol.
The bigger cc do have more touque but you dont need that in a 2wd you need higher revs that will give you more top end, Its a balancing game between revs and touque the more you have of one the less you have of the other. for me i wouldnt go bigger than a good 26 in a 2wd .