I will try to give examples of each, reviews, tips, and where to purchase.

Safety - Motorized bikes travel at higher speeds than a regular bicycle safety should always be a concern Helmet, gloves, coat, lights, good brakes,

Legality - Federal law states under 50cc and 20mph. Local laws vary.

Frame, rack or trailer mount - Purely a matter of choice frame or v mount gives the best handling characteristics, rack mounting can be used on most any bike, trailer mount allows the option of keeping your bicycle original.

Chain, belt, friction drive, push trailer - these to can be a matter of preference

Chain probably is the most durable with no slippage

Belt is lighter and quieter though you may experience slippage with a smooth belt not so with a toothed belt

Friction drive is the easiest to install and the lightest but you will want to use a smooth tire and wet or snowy conditions will cause slippage

 
2-stroke, 4-stroke, electric -

2-stroke is the simplest of engines but will require pre-mixing of gas & oil

4-strokes offer ease of operation for most users no mixing of fuel wide power range long life

Electric motors are quiet have high torque and can be run with different voltages from 12 to 72 volts

The main drawback is batteries SLA’s are weighty only last for 200 to 300 charges

Nicad’s offer greater range with less weight but are more expensive 1000+ charges

Lithium polymer batteries are the best 3000 charges but cost is the killer here, low weight high output

Chain saw, weed eater - these options are for the more inventive type

MPG - this is and area with a lot of discrepancies much depends on your type of riding, I personally average

120 to 130mpg in the area I live with a 4-stroke grubee kit

Forums: 

 http://motoredbikes.com     Motored bike forum
http://motorbicycling.com  Motorized bicycle discussion forum. Our topics cover everything from motorized bicycle engine kits, repairs, building and performance

http://forums.motorizedbicycleracing.com/

http://visforvoltage.org/

http://www.ebikehub.com/

All sites that have been reviewed will have a 1 to 5 rating with 5 being the best/recomended
3 average 1 buyer beware

Price ratings A, B, C, A- best quality for dollar. B- average quality for dollar. C- poor quality for dollar.
First number site rating, second quality for dollar

List of suppliers:

Colorado ez motorbike dealer, Grubee 2 and 4-stroke engine kits call 1-719-268-6639 Sean 5, A

Motorized Bicycle SoCal  Grubee 2-strokes, ez motorbike, LA Calif. Neil 714-341-7998 5, A

Bicycle Motors gasbike.net Grubee engines, 80cc engine kits, 2 and 4-stroke engine kits, parts, bicycles, service department, articles, monthly promotions 4, 2

http://usabikemotors.com/  These guys have the best Grubee engines around. Top rated, friendly, recomended 5, A

http://ezmotorbike.com New company with new transmission now being tested by dealers. Kits for sale now $650.00

http://grubeeinc.com    2 & 4-stroke kits wholesale. China

http://bicycle-engines.com   Exclusive distributer of the Grubee engines 4-stroke, 2-stroke kits, parts. Montana 3,B

http://bike-engine.com  80cc bicycle engine Fake EPA. Colorado

http://www.simpsonmotorbikes.com/    grubee engines, whizzer bikes, exmotorbike owner Sacramento Calif.

https://www.zoombicycles.com/   Canada

http://www.zone8cycling.com

http://www.kingsmotorbikes.com/ Same as gasbike

http://www.fiveflagsmotorbikes.com/  Grubee 4-stroke kits, electric motors. Florida

http://www.thatsdax.com   The Titan 4-stroke, 2-stroke kits and parts. Colorado

http://www.spookytoothcycles.com
Electric and Gas Powered Bicycle Engines, Motorized Bicycles, Bicycle Engine Kits, Bikes and Bike Parts, and more! AZ

http://www.sickbikeparts.com/  Shifter kit, misc. parts

http://www.bikeengines.com/  Gebe belt drive

http://www.americanchaindrive.com/

http://www.staton-inc.com/  American made gear box. Staton-Inc a world class manufacturer of motorized bicycles, bike engine kits, gear drive kits since 1984. Oklahoma.

http://www.chrishill.myknet.org/ Chris's upgraded 2-strokes Canada

 http://www.derringercycles.com/ custom bicycles, 2-stroke engine, gas motorized bicycles. California

http://www.austingreenbikes.com/  2-stroke kits, motor assisted bicycles. Austin Texas

http://www.zbox.com/  2 & 4 stroke kits Austraila

http://www.savegasmoney17.com  New site complete motorized bicycles. Watch for the free viral pop up!

More listings comming

Electric motors:

http://www.electricrider.com 

http://www.werelectrified.com

http://www.falconev.com/ 

http://www.poweridestore.com

http://www.goldenmotor.com

http://www.evdeals.com

http://www.optibike.com/

http://www.electric-bikes.com/

http://www.comcycle-usa.com/

http://www.bionx.ca/