2-stroke expansion chamber and "street tune" does not compute, in the 1970's there were almost zero noise and pollution laws, this is no longer the case and anything you buy or build in the way of an expansion chamber is going to be for track use only.
... holy you raced hardtail just to save weight that's hardcore, the bike must have rode and handled about as good as a hay wagon.

 

In some of the old threads they talk about Scott Clough in California for specialty work on 2 strokes. He has made exhaust for other 2 strokes.

 

So maybe start with Toomey and see if they have patterns for your Yamaha or if they can do a custom design:

Toomey Racing USA

Jemco still exists as well:

EXHAUST SYSTEMS (IN STOCK) - YAMAHA - Jemco Exhaust & Performance

Jim Lomas is also still around:

JL YAMAHA EXHAUSTS

I also found this place when searching:

2-Stroke Exhausts — Performance Fabrications

Sadly, Swarbrick racing has a banner on their website saying they are no longer able to accept orders. They were my go to for Suzuki stuff. I wonder what happened to them?

You might be facing a make your own situation these days unless you can stumble onto a used set of chambers on Ebay. Anybody know if Roy Bacon's book is still considered useful these days?

Amazon.com

Get the cones from Cone Engineering:

Exhaust Conical Steel Products, Conicals, Tips, Steel Megaphones, Transitions, Reverse Cones

Personally, I think I would fix up a set of the C model high pipes - they look pretty chamber like anyway:

Yamaha 200 CS3-C Scrambler OEM Right Left Exhaust Muffler Set 1971 PA AP-408 | eBay

Good luck and I hope this helps!