cracked case
 

while changing front pipe i cracek the case not to bad how will this effect my bike and how can i fix it ??/

Can you take a picture?

When you say crack, do you mean the cylinder head has a crack?

if that were the exhaust goes in then yes bike still statrs and doesnt leek oil or lose pressure

If it's just the ring around where the exhaust pipe fits, then it won't be a problem and yours is among many other SVs that have this 'feature'.
The end of the downpipe is a flat flange which seals against a similarly flat section of the head, as long as that bit isn't damaged it'll be OK.