Rene,
  Sorry mate, I tried for a while to get info on this for myself to no avail.
  I was assured by hotels and such that scooter type rentals are possible, but the closest I came to a lead on a bigger bike was to rent one in Pattaya, and ride it to Issan.
  That isn't to say that you won't find one once you are there, but I wasn't going to leave that to chance.
Cheers,
Daewoo