Building a 4-stroke bicycle

That was the spare sprocket that came with the 4g transmission.

That photo was for reference if anyone needed it.

I know that is the 9T sprocket, and the default one installed on the transmission is 10T.
The issue you will find is there are few sprokets like this that will fit a 415 chain that comes with these kits. The only company that has a very wide selection is Stanton Inc. They actually take the common sprocket and machine the thickness down to fit the 415 chain. Don't bother looking anywhere else.
I have messaged them. Just waiting on a response, and hopefully avoid getting the wrong item.
He calleed me back, but not on the weekend. If the sprocket states 5/8"bore for a #415 chain you found the one. Just choose the number of teeth you want.
This looks exactly like what you want.

Thought I would drop by, and show my final setup. Very fun to ride.

I've only done two or three test drives to make sure everything is working correctly.
I have been looking to upgrade the exhaust. I was not sure what would fit my engine though.

I have this one on a watchlist on Ebay.

Any recommendations anyone?
Since 4 strokes have valves, exhaust back pressure isn't as big of a deal. Due to the way your engine is facing wind force will constantly be trying to enter your exhaust while moving foward. The ideal exit for your exhaust would be with the opening behind you. This will also lower how loud the engine sounds while you're moving. It'll also keep you from breathing exhaust fumes while sitting idle.

The exhaust you asked about does look like a nice one.
The product listing said it was for a two stroke, but their description says it's for a 4 stroke or 2 stroke 🤔