PLEASE do yourself a favor and check into using a quad or 4 wheeler as an engine donor. It comes with reverse, you can use an air cooled one for that size buggy and the power would really suprise you in something that small.
A 300 ex is an outstanding little quad motor and you should be able to find a complete quad for 1000 to 1500, you could use the front end, the motor and electrics and sell off some of the rest on flea bay to make most of your money back.
And you will have a hard time killing that motor. We use them here in smaller buggies like the one your considering. It's a proven formula that works everytime.
Here's a link to the video section of this site with a 300 ex in a chinese type mini buggy. small, fun, CHEAP and dead reliable little engine. not to mention you can find smaller used 4 wheelers of just about all types on craigslist.
http://www.dtsfab.com/index/index.php?topic=1465.0If it's your first build I would really recomend keeping it as simple as possible, and 600 streetbikes motors are just about anything but simple. Best of Luck, snoopy