oil at valve cover
 

I got a spiderbox 150 buggy a little while back and noticed that there is oil at the bottom of the valve cover, does this mean the gasket is bad? also could this cause the kart to smoke?

Yes to the first question, I would clean it off with some brake cleaner and let it run to pinpoint the leak.

The second question, we need more info. Does it smoke from the tail pipe, or the engine? If the valve cover is leaking bad enough, it will run down the head and end up on or near the exhaust, which will smoke from the exterior of the engine. If it is from the tailpipe, then you might need to look into a set of piston rings, as you have some 'blowby'.