2 stroke Vs. 4 cycle-
Personally the more I've used the China Girl 2 strokes, the more I like em for a bicycle motor-
1. Cheaper initial cost and parts
2. They fit between most pedals- have to go wider crank for a heavier 4 stroke that carries oil internally and needs valves beside the head.
3. on the negative, they are a little noisier I think, but noise can be minimized
4. 2 strokes need less "power conversion" apparatus. If you planned to use a shift kit to take advantage of the bicycle gears with the motor, this might equal out some, but you can run a chain straight from the 2 stroke to the back wheel gear, no fuss no muss.
I have a scooter, with 16" moped size wheels even, but I find my motorized bicycle so much more convenient to use, and part of that is that I like to cut the power and pedal around. after a long racing career I keep the seat up at normal peddling height and have narrow 700 C and 27" tires so that I have a fairly viable peddling bicycle otherwise- and it rolls and rolls for blocks after I cut the motor off.
Being in Florida too you'll probably have no problems with the authorities, but please don't be a maniac. as to your query about speed, when you really go at it and if you want to use a bike for practical transportation, you may find that a decent cruising speed is more what you're after as opposed to the top end- The little motors scream quite a bit wide open- With some bigger gearing (that means smaller sprocket in the back) you can be at 25 or 30 without opening the motor up full and taxing it so- it's your motor and you don't want to repair it all the time right?
I've had good results with a 36 on a 66 motor, but a 50cc handles a 39 pretty well and still moves along nicely, with a lighter and quieter, but obviously less powerful motor. See my sound comparison video in the video threads
If it were me, and unless you're a super motor mechanic, I'd start out simple and stay to a beaten kit path whether 2 or 4 stroke- I've paid more attention to the bike itself although I do have two builds with both sizes of China Girl 2 strokers- Wheels that roll and tires that don't vibrate like beach tires do are big fators for me- I use shock seat posts and have a shock fork on the faster bike.
I like to use alloy BMX bars with a slight rise for my bad back but most importantly that have a solid perpenicular grip on them- ya gotta hang on!
A billet intake really helps the motor perform as intended as does a newer 66 speed carb when using a newer 66 motor.
My bikes are lightweight so I can push them up and down a flight of apartment stairs, but I wouldn't have the any other way- that's why I like three piece alloy crank conversions and the alloy bars and stems, and always alloy rims for the relatively smooth Florida streets.
A 415 TRIKE chain weighs little more than half the standard kit chain, although the same width, and really rolls quietly- a real difference peddling too.