All the mopeds I've seen come 50cc stock, but you can buy cylinder kits to make them 60 or 70cc. The reason they're 50cc is because most laws regarding mopeds require them to be 50cc or less.
I've seen mopeds built to do 50 mph, maybe more.
Mopeds are lighter than most scooters, say one of those new Kymko scooters.
Price for a used ped: $25 (not running) to $800 (restored vintage or slightly used.)
Price for a new moped: $650 for a very basic moped. 1500 for a nice, relatively fast moped.
Two-stroke and four-stroke engines have their advantages and disadvantages. A two stroke builds more power per cubic inch (or centimeter) but isn't as fuel efficient (newer two stroke 50 cc engines still manage to produce 150 miles per gallon in some cases). Plus, older two stroke mopeds requre a pre-mix gas (gas and oil mixed) Most newer two stroke mopeds have an auto mix system, where you fill the main tank with gas and a smaller tank with two stroke oil and it automatically mixes in the carburator. Most mopeds are two stroke. I think the only four stroke mopeds were built by Indian back in the 70's.
Horsepower is between 1 and 2 usually, but I've heard of two stroke 50cc engines being modified to build 14 hp. Again, most laws require that mopeds be restricted to 2 hp or less. Torque, I'm not sure as far as foot-pounds go, but acceleration-wise, a one speed will accelerate slowly until you hit about 15 or 20, and a two speed or a variomatic moped has excellent acceleration.
But, one thing to keep in mind, moped laws are built around mopeds and mopeds are built around moped laws. Most states requre mopeds to be 50cc or less. Some states have other laws such as: must have working pedal system; must have no more than 2 hp; must go no faster than 30; must have fully automatic transmission. If a moped fits your states criteria, you usually get such advantages as cheap licensing and registerring (if even required) and cheap insurance (if requred.)
For example, in South dakota, If your moped is under 50cc, doesn't go faster than 30, has working pedal system, has headlight,brakelight,and taillight, and is 2 hp or less, you don't need license plates or stickers, registration, or insurance. All you need is a class A driver's license, which is just the basic car driver's licence.
Hope that helps.