I have been to Laos several times, and I have never seen a sports bike there. It's a wonderful country and absolutely fabulous for off-road riding, and hence most rentals have XR250/Baja/KLX on offer. As mentioned Jules and Jimoi are both great guys that will be able to help you with that.

If you desperately want a sports bike, imho you will have to rent in Chiang Mai. It's a little complicated and requires some paperwork (e.g. letter from the owner), but it can be done. The one guy in Chiang Mai who regularly arranges this is Dang, but he does not have sports bikes. The next best bet would probably be Gung from Mr Mechanic, and if that fails ask Tony from Tony's big bikes, I think both of them have 250 Ninjas and ER6's.

Also, have a look at gt-rider.com and the forum there, maybe someone can help you out.

Come to think of it, Rt 13 in Laos is great, but Chiang Mai - Mae Hong Son and the minor roads in that area also have a lot of twisties and beautiful scenery. If you haven't ridden that yet, that may be a lot easier and you'll also have a great time.

I suggest too, to us rent a bike in Thailand, and do the paperwork for using it in Laos.

You should be able to get a insurance on the border. They looks pretty crazy - at sample for cars; the insurance covers only 2200 USD on the oposite car. But for 2`200 USD you can repair much in Laos ;-).

I would always suggest an additional insurance for your health and for your equippement - when you traveling far away.

In Chiang Mai you get that well equipped bike you search for, and too additional equippement..

cheap place to stay in veng vieng
 

We have spent a night here in a place that was very friendly for bikers, the owner told me to lock the bike twice so I started to thing that it might be unsafe to leave the bike outside. He said it was ok to bring the bike inside and lent me a chain to lock it so I was able to sleep knowing that the bike would be OK. The pace is called someday guesthouse and is just on the edge of the town before main junction, free wifi, coffee in the morning, hot shower and clean room. Head into town for some good food, we found a Mexican called amigos which did good fajitas to make a change from Asia food. :)