i'll tell you this, i have a 50 c.c. slant head motor on a 20" frame with 20" wheels. My 50 c.c. can catch and pass my friends 24" bike with 20" wheels and he has an 80 c.c. motor on his. the 50 c.c. revs higher than the 80 c.c. but it lacks the bottom end torque that the 80 c.c. has.
yes, mine has the carb bolted directly to the cylinder, and there is no intake tube. the down side to this set up is that it will take a lot of work to get the carb off because it's mounted to the cylinder with studs and nuts. the only way that i can see to get the carb off would be to drill out the studs (where the nuts are) big enough so that the nuts will fall off. the nuts sit in recessed pockets on each side of the carb throat and there is no way to get a wrench or socket on the nuts to take them off.