You can use just about any small petrol, or if you can find one, diesel motor for a portable charger. Lawn mower motors are a good choice, 4 stroke is preferred, though 2 stroke can work. If you took the cutting plate off, and now find the motor hard to start, you will need to add a flywheel. A large steel pulley will work.

Weed Eater or Whipper Snipper motors? Forget it, your going to need at least 4 or more horse power to drive a alternator at full power.

This is my first build of the portable charger. The motor is a Kubota 5.2hp 200cc 4 stroke I bought off ebay a few years back. The alternator is a 75amp 12v Bosch with internal regulator. At the time I had access to a laser cutter, so I lasercut the adapter plate from 3mm mild steel. It worked pretty good, but wasn't very portable.

Since then I've mounted the charger to a old trolley, see the photo above. Now its portable.The wheels are from an golf buggy.

After building the charger I discovered the motor runs in the opposite direction to the direction the alternator was designed to run. I should have mounted the alternator out the front of the motor, facing the other way, but too late now, and besides I like the compact layout. The only problem with running a alternator backward is the cooling fan wont work as good as the alternator needs it to. If you only intend to use the charger for a few minutes, on light loads, you can get away with it. But if you want a bit more reliability, then you need to reverse the fan.