Your engine performance wont be effected but then again what is performance if you cant steer the boat?..
You can get a new piece welded on pretty cheap..last I paid for mine 200 bucks i think..
there is a bolt on type that i have heard people use..from what I gather its also a very cheap fix but some swear by it...cant recall the name of it..google it I guess..
If more than half is gone.....it will affect the steering significantly.
That skeg is a key component in the steering dynamics of a boat....much like a skeg on a surfboard is necessary to "bite" the water and prevent it from sliding in a wave.