In The Wind

My friend, Jill got herself an old starter scoot- A TX 500- and it has head gasket leak. In tearing it down for her I found rubber O-Ring type seals on both sides of head gasket, on the cyliner case and the head.
Where can we get the replacement rubbers?
Are those rubber dealies reliable or are they notorious for failing?..I somehow hold that design characteristic in contempt as they are not continuous, but are instead only on the outside ends of the cylinders.
We'll be watching and waiting for all your thoughts on this.
Thanks ITW