You might consider a simpler approach, such as giving students two quizzes of 50 questions each and keeping the highest score, or something else.
Otherwise, for your initial approach, you can work on the server side or on the client side.
On the client side, it is preferable to develop a plugin, so you don't have to reimplement your changes with each update of the Moodle package. Other people can help you with this.
Client-side work is for users like me without access to the server side.
As I mentioned earlier, due to lack of time I haven't tried what I suggest (which I don't usually do). So these are just suggestions that may or may not work. It's up to you to try.
... the total number of marks is calculated from 100, but I need calculating marks from 50.
Use "calculations" in the "Gradebook setup" to transform the marks.
... in that summary page has a display user answer of 100 questions.
I think you're talking about the "review" page. If you modify the quiz with jQuery, the changes should also be reflected in the review page.
... the navigation panel shows 100 links.
With jQuery, it's easy to hide the navigation block. It may also be possible to hide specific links in the block.