i can't think of any 2nd hand dealers in Phuket that would answer emails, especially one in English. sorry but i think u need to get down here, rent a bike and drive around as many dealers as possible (loads along Thepkassatri road, and Chaofah East and West). that will at least give you an idea of the ridiculous prices they expect for worn out motors.
then while you're down in Phuket town drive around some of the main streets and you'll see a few cars parked by the roadside with for sale signs. i often see 1 or 2 parked outside Tescos on Yaowarat rd, esp. weekends. if u see something like this:
give them a call, offer them 50 grand and take it from there.