my bike is running funny

if it's the exhaust rattling then it could be a little bit lean at the fuel mixture.

did he bore it out when he rebuilt it?

sometimes it's necessary to adjust the fuel/air mixture screw.

you could try removing the airbox lid. ride it a little see if it is worse or better.

if it's worse then you are a little lean and you need to turn the fuel/air mixture screw 1/4-1/2 turn to the left

if it gets better then it is too rich turn the fuel/air mixture screw to the right 1/4-1/2 turn.
 
if the exhaust is rattling, maybe check the bolts to see if theyre tight. and on my fmf where the pipe comes out of the cylinder, there is a spring there