Dont like your chances, but it could be arranged, and only if you are prepared to pay for the shop to pick up the bike in Bkk & then transport it back to Chiang Mai.
Alternatively why not rent a bike in Pattaya & return it to Pattaya?
Or why not start & finish in the North where all the good riding is?
As a rough guide the riding south of Chiang Mai is not great - hundreds of kms of flat rice fields, which tend to get a bit boring after a few hours.
Davidfl
Keep the power on