Bicycle  Conversion
Most of us have seen a motorized bicycle by now in 2020. These little motors were first built in the USSR but the Chinese realized their usefulness and began to manufacture them to export all over the world.  In third world countries they are a practical solution for mobility, but in other areas a recreational joy.  Simple, affordable, and with fuel mileage up to 170 mpg.
In the post WW2 years the Soviet Union produced the D series motor for their people who needed transportation about their towns and villages.  Millions of these were built back then.  By 1970's China began to redesign, refine, and produce these.

The Chinese Motor is readily available today as a kit with everything you need. Some are 49cc others 66 to 80cc and now 100cc are showing up.  The kit weighs about 25 lbs. Each State has different laws as to motor size and speed these bikes can be operated.  They are more dependable nowdays.

Electric motor kits are also available.  They come in various watts of power and the motors are generally mounted in a front or rear wheel and ready to install. These are usually more expensive than gasoline but are easier to install and are cleaner to operate.
These kits can be found easily on ebay or on bicycle motorization forums where banner ad's are displayed by numerous small businesses which deal in them. There are various prices and features. Recoil starters, centrifugal clutches, expansion chamber exhaust, and kits for multiple gear systems. 
Each State has it's own laws regarding converted bicycles.  Some refer to the motor as a helper motor. The bike must be able to be pedal powered by the rider.  Should be less than 1hp if gas and have only one gear. If electric, it must conform to that State's power limit. State's have different mph speeds that these bicycles can be operated. Many do not require a vehicle operators license or insurance. The bike will need to have standard safety equipment, reflectors, lights, and the owner can install accessory options such as turn signals, blinking red or yellow lights facing the rear, etc.  
A basic finished bike without the motor chain guard installed. A comfortable seat is an important addition. Baskets can be installed like saddle bags on the rear wheel and on the handle bars. Below are links to helpful Forums etc.
If you are living in the hills and need more than one gear there is a solution. It isn't legal but...  It will power your multiple gear rear axle and make it so you can get around and still pedal the bicycle too.  See below.

A Jack Shaft kit will allow the motor to transfer power thru multiple gears of the rear axle.  Whether a 2 speed coaster brake axle or 6 speed derailer axle. These are not considered legal.  But if you live in an area with hills and have some flat roads too, this may offer a solution. You can select a lower gear that will allow you to ride up a steeper incline slowly or a higher gear to travel on the flat road at the legal speed of your state.  One of these kits will usually cost more than the motor kit.  The argument is that you only have one gear ratio in use at any given time, using it to adjust for the per cent of uphill grade thus enabling the rider to travel uphill. Or with the proper gear selected for a flat road.  Generally a bicycle is not safe at speeds much above 30 mph.  You can get a lot of money invested in one of these so consider a good locking cable to secure it when parked.

An example of a bike with plastic shopping baskets mounted three on the rear wheel, and one over the front wheel.  The motor is another Chinese kit using a four cycle lawn mower type motor.  There are numerous motor kits available.
There are other ways to modify bicycles as motorized platforms.  The motor could be built into a motorized wagon to propel a bicycle, or the bicycle be modified to contain a cargo bed with the motor in the frame like a pickup truck.  If one were on the road the bike with baskets may be best but if camping or boondocking for a longer period a cargo wagon or bike may be best. A cargo wagon could be rigged as a tent camper also. 

These images are from some time ago.  Today it may be easier to build them as electric bicycles and use solar panels to recharge their batteries.  A full canvas cover would be a good option for a motorized bike along with a good cable lock.