make sure you get some bushings to cushion the A arms, whether you buy em or make em out of whatever rubber/hose you can find. its gonna make your ride better, prevent the metal from rubbing, and keep everything from shattering if you hit bumps.
is this gonna be a street kart or an offroad one? one or the other will affect ur engine shopping alot, the offroad type (atv and dirtbike) will offer more torque but shake ALOT at high rpms. the street type motors will offer less torque low down but higher top end.
also if youre gonna go out and spend the money on an ATV, at least use the suspension from it too. poorly made A-arms will break the first time out and then you're stuck and back to square 1. what kind of metal are you using? what thickness round tube? etc..
lots to think about..