Here is a picture of my fan belt showing the part number. I don't recall where I bought it, but I've had it on for 12 years and it fits the powerhouse fine and still looks new. When the number starts with 42- you'd sure think that means a 42" belt, but it doesn't. I'm sure it was a 43" belt. If your's is too short, then try to find one an inch or even a half inch longer.
Are you using the Model A timing cover? I think someone said the Model B has the generator mount located a little off from the A.
Go to your local Napa store, and tell them you want to look at the Napa Belt Spec. book. Find your top width in the charts and look down and see what lenghts are available. Get a couple belts that are close to what you need, and return the ones that you don't need. I work for Napa, and we do this all the time with old cars and industrial engines.
BTW, Farmall tractors and old Chevy 6's use the same belt as the Model A. I learned that when my 1949 Chevy 1 1/2 ton truck broke a belt near a tractor repair shop. The owner grabbed a Farmall belt and told me they were the same. Lucky for me because I was 200 miles from home.
The 45" belt, with the same profile as a Model A belt, came from O'Rielly auto parts. It wasn't in Camino, it was in Cameron Park, about 20 miles West. I'll bet any O'Rielly Auto Parts would have them.