Asking previous advisor for a better letter of reference
You can certainly ask, and might be successful, or not. It is impossible to say. But a better approach might be for the scholarship people to ask the professor for a better letter on your behalf, explaining the reasoning and perhaps the fact that his is an outlier. 

Having an advocate may be more effective in a situation like this. Another professor might also prevail upon your advisor, but it would be impossible to hide the fact that you'd seen the letter.