That looks like the standard Chinese 2 cycle motor kit. Try checking ebay or some other online auction site if they are available to you in Greece. Look under "Motorized Bicycle Kit". I'm sure they are available worldwide.
I want to know exactly where these guys are riding to get two hundred miles to the gallon. I ride the smallest engines possible and get like 90 miles per gallon. Of course I don't have a pancake flat track indoors out of the slightest breeze maybe that is the difference.