Hey there, I'm on a similar journey right now. Started in Hanoi some months ago, am in Bangladesh now. Few things to consider, off the top:

- 250cc is a double edged sword. Yeah, it's nicer/stronger, but good luck finding any parts or a mechanic for a 250cc bike, especially somewhere in the countryside. In many places this will be next to impossible. Also - many countries will refuse entry with a 250cc bike. Logistically - 125cc is your best bet.

- China is pretty much no go by bike. You won't get a foreign bike in. Unless you go to China, buy a Chinese registered bike and drive it from there. And yeah - of course if you want to go with a guided/scheduled to the hour tour, then you could do that, but i figure it's not what you're after.

- Myanmar is no go with a foreign bike. You gotta book an extra expensive tour, pre-scheduled to the hour, with a guide. It's your only way to get in. I tried every possible way - no go. Your options would be: ride across the river from Thailand to Myanmar, leave your bike in the bushes, then cross to Myanmar bike less and hope your bike will wait for you where you left it. This is far from a perfect solution but if i could have a second go, i may actually have just tried it. The river was so shallow, and i could literally throw rocks into Myanmar. Highly doubt anybody would discover me but then again who knows. And - you'd have to do the same amount of sneaking around on the other side later - which again is far from ideal. (West of Myanmar, the police are no longer 'charmed' by white faces as they are in SE Asia). Your other option is to ship the bike from Bangkok to Kolkotta and meet it there. Do NOT ship to Bangladesh. I made the mistake of doing that and spent the last month fighting with customs. It was a nightmare which involved the foreign minister, my embassy, the minister of finanze and a whole chain of other politicians. Paid more im bribes than my bike is worth, it was a nightmare.

- India you cannot get your bike into unless you have a Carnet De Passage. Vietnam does not honor nor issue the Carnet. Closest place that would do it for you is Kuala Lumpur. But they demanded a list of ridiculous things from me such as "proof of property ownership in malaysia" to give me a carnet - i've never even been there so it was obviously not possible, and i'll find out within 2 weeks if the docs i have will be sufficient for India...

Anyways - It's an epic, amazing journey so far for me, but the bureaucracy all along the way is a MAJOR struggle. So surely brace yourself for that! Good luck!