I've just replace the secondary drive belt on my Kity. I could measure the length, but when I bought a replacement from my local motor factors, although it was the right length, I couldn't get it on because it was too thick, there wasn't enough clearance between the pulley and the cover. The original was flat, this was a V.
So I took it back and they rummage around and got me something that was flatter. Not the same as the original, but near enough. I've fitted it and it works perfectly, and because it is considerably more substantial than the original, I'm hoping it will last for ever.
So check your local motor factors, you may be surprised.