Well..... I decided to try to answer a few of my questions....
First, I removed the muff and looked into it from both ends-- it has several walls inside and the chrome outer is a skin, so I'm glad that I didn't just take a cutting disc to it. No, it can't easily be hollowed out and the cut end can't be finished.
Next, I took it for a ride to check for performance/drivability problems.... Although very loud, it seemed to have 1/3 more power and performance... and a decel backfire that sounds like gunfire!
After that, I measured the headpipe for an aftermarket muffler.
The pipe OD is 2" and necks down to 1 5/8" and has a 1 3/4" ring around this smaller part. Farther back on the 1 5/8 part a 3/4 wide strip of brass foil was wrapped around the end of the pipe like a piece of electrical tape to a thicness of 1 7/8" diameter. I made a slshcut straight out of 1 3/4" OD muffler pipe, but am unable to tighten it at present. I think I'm going to make another straight pipe out of 2" ID muffler tubing. I'm going to hafta install some kind of muffler on it before putting my son on it so he doesn't get into trouble, but for now....