Unfortunately traversing SE Asia is no longer an easy process as it once was, you didn't state where you planned to enter China but whichever entry point you will need a guide and a Chinese driving license (although an IDP may suffice). i would suggest you contact Jah @ Ride China who can give you all the correct information and costings on the China leg.
China Motorcycle Tours, China Motorbike tours, Bike Rentals & Services
Reaching Boten in Laos, you may be able to enter on a Temporary import which would give you a maximum of 30 days in Laos (you must request this at the time of entry if required 15 days being the standard period). It is also possible you may require a guide or at least pay for some paperwork from a tour company to travel, Jah @ Ride China may be able to help on this.
Exiting Laos to Thailand, up until early last year, overlanders could travel easily through the Kingdom, again times have changed and you will require a guide to escort you to the Malaysian border, i would suggest you contact one of the many Facebook groups dealing with overland travel in Thailand amongst others, things change often and we hear stories of riders crossing through Thailand with no need for a guide of any kind then the next day a different picture .
This being one of the biggest.
Overland Sphere
Maybe some of the members can offer more details, but as it stands it's becoming an expensive process crossing these countries.