a 2 stroke will rev a lot higher than a 4 stroke. a 2 stroke will make more power in the upper rpms than a 4 stroke, plus a 2 stroke can be tuned to hit the power band with an expansion chamber. an expansion chamber (when tuned right) will act like a supercharger. parts for a 2 stroke are generally cheaper than they are for a 4 stroke.
4 strokes do not rev as high but they make a ton of low end torque. you can buy speed parts for 4 strokes and you can get a ton of power out of a 4 stroke but it'll cost you a ton of money (research jr. dragsters & parts for the engines)
a 4 stroke is just like a car engine with an intake valve, exhaust valve, a cam, and tappets(lifters).
a 2 stroke has no actual valves unless it has reed valves (h.t. engines don't), no cam, no tappets, less internally moving parts.
a 2 stroke will generally be louder than a 4 stroke, but a 4 stroke can be VERY loud with the right pipe on it.
4 strokes are generally bigger than 2 strokes of the same c.c.'s.
you can do a ton of work to both a 2 stroke and a 4 stroke, but you will get a better quality 4 stroke for a bicycle. on the other hand, the 2 stroke bike engine kits are cheap and they are throw away engines basically.
you can buy them cheap, ride them until the puke and then buy another one for under $100.00.
you can sink $300.00 easy into a 4 stroke, and then when it pukes you can spend another $300.00 fixing it (or replacing it).
a 4 stroke has oil in the crank case that will need to be changed on occasion. a 2 stroke has no oil in the crank case...the oil to lube the brearings and rings comes from the 2 stroke oil that you put in the gas.
I guess a lot of it is personal opinion for the application.
In my opinion, the cheap h.t. 2 stroke is the way to go when building a bicycle.
Mostly for amount of power they make for the low cost. They also look a lot better in the frame of a bicycle than a 4 stoke does.
Have you ever ridden a 2 stroke dirt bike or a 4 stroke dirt bike?
a 2 stroke will usually walk away from a 4 stroke due to the higher rpms. and the power that they make in those high rpms.
but these days, 4 stroke dirt bikes are just as fast as the older 2 strokes used to be, but there has been years of research and development behind making a 4 stroke dirt bike perform as well as a 2 stroke.