I've just done exactly what the OP asked about
: I bought a bike from Flamingo Travel in Hanoi, drove down Vietnam, through Cambodia and Thailand and am now in Kuala Lumpur.
Cambodia: at Tinh Bien – Phnom Den, no questions asked about the bike, just wanted my passport and 25 USD for the visa
Thailand: got refused at the Poipet border as my bike isn't registered in my name, so I went south to Pailin – Pong Nam Ron, where with a bit of patience (2 hours) I got all the paperwork to temporarily import my bike to Thailand. Here I needed the original registration (blue card), a letter from the "owner" saying I'm free to do with it as I like and my international driver's licence.
Malaysia: near Satun - Kaki Bukit
, stamped out the bike at Thai customs, entered Malaysia without questions about the bike. I did get stopped at a police checkpoint in central Malaysia, but they were happy with my passport and international driver's licence.
All of this is done without any insurance btw.