I can't take credit for calling out the right size, it was one of the regulars on here, maybe DEET or Enduronut, pointed out the correct O-ring size for the VM26SH is 007 (metric).  My bike is a '73 DT3 250.
I ordered a Yamaha OEM ring from one of the major parts houses at $5.99.  It hasn't arrived yet and we had a nice spring day here yesterday and I am not the most patient, so headed off to Harbor Freight to see what I could find.  I now have 20 #007 rings (19 left with one installed) and a whole bunch more in various sizes for $8.99!  Plus, one more OEM on the way...
Not sure how they'll hold up, but supposed to be nitrile rubber, so guessing they will work fine.
Bike had been blubbering above 4K RPM and several members on here advised me to try the O-ring first.  When I got into it yesterday, I had to cut to old one off.  It had basically turned into a hard plastic ring.  I think it was probably original.  I don't think the carb had ever been off the bike, let alone opened up.  And it was nice and clean inside!  Bike started on the second kick and now runs the way she is supposed to, awesome!  Many thanks to the guys (I know Airborne Silva was one) who pointed this out to me!
 

I’ve just ordered one correct Yamaha o ring for £7.50 did a bit of searching and found these extra durable o rings, almost the same size, bag of 50 for £7.00