Gas engine on a robot: Is is possible? Is it a good idea?
Something like this from walmart is a good example of something you could use, and it even has a handy 12VDC output, 1000 Watts max generator with a 4L fuel tank. Please be aware that the 12VDC output only has 8.3A available, meaning it's only 100W. Your next bet to use more of the power from this would be to use a 1000W AC->DC converter (which you would need to buy) to get the most out of it. The 12VDC can go to an automotive SLA battery bank too.

Otherwise, you can try to do it from a motor and alternator:
Just get a small 2 stroke motor/generator, and bolt it to your machine, and the drive train output goes directly to an alternator (you can buy these or salvage from a car). The alternator DC output goes to the 12V automotive SLA battery bank on your mobile machine.

The alternator will charge the batteries, while the batteries and also power directly from the alternator (unless you use some diodes to block this from happening) can go to power whatever you want in your robotic snowblower. 

Any converter you use should be automotive rated, and should be able to handle the crazy noise and over voltage spikes on the output of the alternator.