First of all, sprocket size,wheel size and tire size make a big difference in how fast these 2 strokes will go...49 or 66 c.c.s
also, rider weight and bike weight also play a big roll in the speed.
my friend and i have similar bikes (both are old 20" schwinn stingray frames)
we are both running 20" wheels and the exact same tires front and rear.
I have a 49 c.c. engine, and he has a 66 c.c. engine, and both bikes have 44 tooth rear sprockets.
our bikes went exactly the same speed (about 25 mph) when we first put them together.
(with me making the test runs on both bikes because i weigh the least)
I dropped to a 41 tooth rear sprocket, added an expansion chamber, high flow air filter, ngk spark plug, accell 8 mm. spark plug wire and now my 49 c.c. will blow his 66 c.c. away big time. (i weigh 150 pounds, he weighs about 190 pounds)
we had our bikes clocked with radar, and my 49 c.c. is getting 32 mph top speeds on flat ground with me riding it, and his 66 c.c. is just barely hitting 25mph top speeds with him riding it. his is all stock including the muffler and air filter, but he weighs more than me.
we swapped bikes, and with him on mine, the top speed dropped to 27 mph, and with me on his bike, the speed increased to 27 mph. so rider weight plays a big roll.
my occ chopper, which has a 66 c.c. engine, no expansion chamber(it has a long pipe with a modified muffler) a 41 tooth rear sprocket, and a velocity stack on the carb, will hit about 35 mph top speed with me on it.
the difference is the rear tire size (height).
my other bike has a 20" wheel with a 21 inch tall tire on the back. my occ bike has a 20" x4" rear wheel, but the tire is 24" tall. the tire height adds a little to the top speed.
there are several variables that are involved when talking about top speeds, low end power, etc.
but from what i have experienced, a 49 c.c. and a 66 c.c. will be go about the same speed stock because they are both limited by the gearing.
your top speed will depend on all the variable (rider weight, rear sprocket size, wheel & tire size, etc.)
also, if you buy one, make sure to read which engine mount it has. some are listed as "big mount" and these are specifically for bikes that have a bigger diameter front frame down tube than normal. the "big mount" is only the front mount on these engfines, the rear mount is standard size. but if you have a bike frame where the front down tube is the same size as the seat post tube, do not get the "big mount" engine because the front mount will be too big, and it will not fit your downtube. 1/2 of the big mount is cast into the front crankcase so it isn't like you can just change the mount to a smaller one.
as for noise, they are pretty quiet with the stock muffler. In my opinion (everyone's definition of loud is differnt),both 49 c.c. and 66 c.c. are slightly quieter than a weedeater.