For the engine, you'd look on your local Craigslist. I can send you a link to what I'm talking about, but that wouldn't help ypu much. You'd need to find something similar near you. Otherwise, you can use the super common Predator 212 from Harbor Freight. They usually have a coupon that lets you get one for about $99 but be aware, you would have to break it in yourself. Easy, but takes some patience.
As for the clutch, here's an Amazon link.
The one I sent is for a 3/4" shaft. Be aware of the engine's output shaft size. As a general rule of thumb, 5-6 HP engines are usually 3/4", 8HP and above are usually 1". You can just google "1 inch go kart clutch" to find one.