Marco777
Does not sound or look very good at all.
I think you should have done a bit more research first.
Take a look at
1. Buying a Local Bike
https://www.gt-rider.com/bikes.html#BUYINGABIKE
It is possible to take a vehicle out of the country, if it does not belong to you, but is in the name of another person. To do this you must have permission from the vehicle owner to export it. There is a standard form you can use for this. Click here to see what it looks like.
Under this system then you can buy a motorcycle in another person's name still ride it out "overseas." To do this, you must have signed copies of the real owner's valid / current (not expired) Thai ID card and Thai house registration.
This normally works, however it is open to the duty Customs officer's personal discretion; and if you have a communication problem or a personality clash (I have good experience of this) it might not go down so well. For foreign riders with Thai registered bikes this is a not uncommon problem on the border. One of the Customs concern is that the motorcycle may be stolen, or may not be returned. So, be patient, tolerant and understanding of the system.
Don't believe the shop, where you buy the bike, that you only need a receipt & / or a photocopy of the bike registration. You won't get out of the country, and it is no fun riding all the way to the border, to discover that you've wasted your time and money & probably stuffed up your holiday.
2. Crossing Borders - Crossing With a Bike
https://www.gt-rider.com/crossingborders.html
2. Bike: you need to prove it's "real", not stolen & have valid docs to support this = proof of ownership, the bike licence / registration, valid insurance. If you can't produce these then you're in for a tough time trying to get out of Thailand!
If the bike is not in your name or rented, then you need permission from the owner to legally export the bike. Click here for info on taking a bike out that is not registered in your name. Note that this works 99% of the time, but that remaining 1% is hard to take if you get caught short. You've been warned!
So to sum it up - DON'T like your chances. Get permission from the owner to take the bike out of the country. (And, if it does not have a number plate on it, you need one - get it legal & plated.)