I think hes asking for a 'friend', hypothetically speaking, he may have hypothetically sold it and it may have hypothetically already happened.
Bill of sale is CYA. Use it every time. Also, if you made or modified the go kart yourself, you do incur a certain liability. You can always include a 'release of liability and accident waiver' form that has verbiage in there specifically for what you are talking about.
I'd say if the go kart was stock, or like stock condition, then its on the go kart manufacturer if they injured themselves due to a defect or product safety problem (e.g. hair gets caught in unguarded axle or clutch).
But if the moron is driving it and injures themselves due to stupid driving, its their own fault.
I dont think anything can prevent you from being sued/prosecuted though, but those documents will show that you did your due diligence and did it in good faith so all parties involved understood that the buyer was incurring risk by purchasing and riding the go kart.
So if you really want to CYA, you get liability insurance, bill of sale, waiver, etc, but meh... its just a go kart who cares?