Finally
Now the sprag clutch is removed, and I have the 51 tooth gear. And it's written in the book that the 51 tooth gear had problems, so Triumph changed them for a 53 tooth gear. So I have to buy the new one.
I found out that my wonderfull Triumph Thunderbird 1995 had a broken sprag clutch (in the book I have (Haynes service & repair manual), they call it starter clutch)... And I took the decision to change it myself. I have the books, and the knowledge to do it, so why not. And I will document the operation in this blog.