Can anyone tell me what this cracked part is? I think it is just a plastic piece that covers the throttle cable. Can I just replace this plastic piece? Anybody got a part number?

You are going to need a new throttle cable assembly, it comes with new cables and the rubber boot over the cables. You can't get that stuff separately.
Yamaha part # 5XD-26302-00-00

Or if your referrng to the actual throttle assembly I can't see in the pic which part is cracked. Maybe a clearer pic with a arrow pointing to what your referring to will help.

i cant see whats cracked either

Ok I circled the part that is cracked. There doesn't seem to be anythin wrong with the bike. Curious =)

You need a new throttle cable. Your throttle cable is made up of two, push/pull, cables. When you least expect it the cable will pinch and well... you'll wish you had replaced that cable. Been there done that :o