I have never modified from original. So the carb and muffler questions are out my area.

I watched this youtube video, he has fitted a different carby with a throttle.


Yes, it is better to have new orings with the aim of sealing the engine up, to prevent over-revving and starting problems. Which can be frustrating.

The small oring is inside the starter, see image below.(71)
[Linked Image]


Happy is he who penetrates the mystery of things.