I assume from reading your posting that you're not willing to pay the extra money for the Kawasaki Ninja 300. As you bought/ordered, and did a down payment for a Kawasaki Ninja 250 they will have to refund your money.

Actual the Thai consumer law is pretty specific about this:

“Reserved cars* for sale are regarded as controlled contract (valid since 1th December 2008

The Office of the Consumer Protection Board reviews complains form consumers on unfair car reservation, such as delay in delivery, high pricing, and reserved fee refund refusal when the loan is not approved. The Consumer Protection Board Committee on Contract has, therefore, announced that this business shall be c controlled contract. It is necessary to describe the brand, model manufacturing year, additional installed devices, benefits, amount of down payment, price, etc.

Moreover, the consumer is entitled to terminate the contract if the business is in breach of contract. The reservation fee or deposit shall be returned to the consumer within 15 days.

- Any business which fails to deliver the contract having contract terms or contract terms with the correct form or fail to deliver the receipt of payment containing correct particulars and statements to the consumer within the period of time shall be liable to imprisonment for a term not exceeding one year or a fine not exceeding one hundred thousand Baht, or both.

- Any business which delivers the receipt of payment containing the amount which is more than the amount actually paid by the consumer and having already received such payment shall be liable to imprisonment for a term not exceeding one month or to a fin of five hundred to ten thousand Baht, or both.”

* Cars and motorcycle sales are covered under the same rules