Let's start with your first question, the governor is important if you intend on young children riding it, if you are grown enough to make decisions for yourself and you are selective on who rides, I don't have a governor in any of my karts other than my kid's kart. The governor's only role is keeping revs down, without it you could possibly blow the engine if you are always running top rpm for long periods of time, but I clip along 30-40mph on a 212 clone and it's just fine.
Important impliments would be a kill switch, make sure everything is straight and symmetrical, and just as much whoa as go.
BRAKES (not being a jerk but I'd be leery of following directions from someone who cant follow direction, or cant spell brakes) I'd run at minimum a disk brake from a mountain bike, preferably a mini bike on the front, and a band brake for the rear, the band brake on my live axle will lock the rear if needed and it's a 25% shoe at best.
Tire size I believe most use slicks, I think they are a lower profile on a 6" wheel, deflated for the PVC to fit over, then inflated to hold tight.
Engine placement I would think centered may be best, if all the weight is to one side it may slide better one way than the other.
A mini bike throttle and killswitch kit is super cheap on Amazon, think mine was $11, and the best source for the steering is commonly an old bicycle and cut the entire neck and front end off.
