Jim, Razor and Ernie, I made myself at the lathe the thin wall tube from an aluminium ISSP 1 5/8" tube (you can buy this tube where you can buy steel, aluminium, brass for a workshop, this kind of pipe or tube is used for high pressure applications and is seamless).
The original ISSP tube has an internal diameter of 1 3/8 " so you may need to bore it to 38 mm internal diameter and for each side you may need to bore to 39 mm, 1/4" deep to make the front and back plate of the TH muffler fit o.k.
Then make the external diameter VERY CAREFULLY to 40 mm, that´s because you have a thin wall at the tube.
What is very important to make this mod. work fine is that you also may need to make the pressure fitting hole a little bit bigger, I´ve opened mine to 2mm to get more pressure for the tank, if you don´t make this, you will have problems with the fuel supply airborne resulting in a dead stick (it is my experience!).
Finally, you may need to buy 4mm threaded stainless steel rod and cut it to the new length also you may need a couple of security nuts (those nuts have a piece of nylon in one side) to fit it back again.
I would like to send you all a sample, but it will be expensive to me to send you this from Mexico.
I will post a picture of it when I can get it.

Regards.