No you don't want to use a rubber belt, those things are one of my pet peeves. Though full disclosure I have one machine with a rubber belt on it. It came to me with it, and the pulley isn't the original and is held on by a huge screw. The belt isn't tight and the machine works great so I'm not gonna mess with it.
Your pulley doesn't look(from the angle of your pic) like a V pulley, it looks straight sided so a straight sided normal lug belt should work just fine. Most of these old machines have some adjustment for raising or lowering the motor a bit so it's not always imperative to get just the right length, more important is to make sure the width of the belt will fit your pulley and hand wheel groove.
Cari