Welcome to the forum, Davi.
We need some specific questions to offer any help. What are you having problems with?
You might consider using the search feature and looking for bike builds with your engine choice. Simply type in 'Honda CRF50' and see what others have done. You can also check out the DYI section where you'll see many builds using non-kit power sources.
Start here> http://www.google.com/cse?cx=partner...F50&gsc.page=1