You can get two of them from this eBay guy with free shipping.
I did the hardware store o-ring and it was a real tight fit. Had to use the bowl screws to draw it down.
Decided to get the proper ones this last time I changed them.

 
 

Not to get too personal, but how much do you weight?
I am 250 and run out of power on 360 and 400 around 55 on an open road with some slope and wind

Thanks RT325. I actually just ordered it from Partzilla, along with a new 150 main Jet, and a new O-ring for the front of the carburetor. I was trying to order a 160 main Jet as well, but every time I added one to the cart, they added another 150, so I figured they may be out of 160s. This old 150 may be varnished up, and I'm pretty sure my old #7 O-ring is shot, and I can see how that could make a big difference. That area needs to have a good seal.

I also received a 17 tooth countershaft sprocket today, but I'm not going to put that on until I get this other issue worked out.

I really appreciate all the help and suggestions!

Msavitt, I think I mentioned earlier in the thread that I actually weigh the same now that I weighed in high school, when I used to ride another CT1 all the time, 182. It's funny, unlike a lot of people I actually lost weight during the pandemic. I was around 205+ a couple years ago.

The test area I've been riding around is also level. It's probably the main jet and that O-ring, after everything I've tried. The bike runs beautifully other than when I'm trying to go wide open throttle in 5th gear, then it sputters at 5500. It's not running out of power, it's just running erratically when it gets to that RPM, like its starving for fuel.

Try a 170 - 180 main jet. 150 - 160 is not enough fuel imo.
Also your air filter limits flow, use a velocity stack with panty hose stretched over the opening.
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Thanks Schu! I just ordered those and canceled the Partzilla order.

I don't have the stock air cleaner box, I'm using one of these. I may try some larger jets, but I'm going to wait and see what happens since I cleaned out the 150 main jet today.

Hmm ok, you could be onto something with a partially restricted main jet varnished up.

Tom P - sorry I missed that impressive detail, sorry was just speaking from personal experience!

Those hard pod filters can wreak havoc on two strokes with resonance ...  also less restrictive so it will run lean...  plus they let dirt in. 

Try a UNI pod filter instead.  The KTM 50 one suggested by Dave Meis works well.Just don't overoil it with sticky oil.  Part number NU-1409ST