Felipe,
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As far as your bike, it looks to be in real good shape and all original as far as I can see. It's always best to try and keep the bike as original as possible when your dealing with vintage bikes. The motor looks pretty clean and original, so if it were me, I'd just go through it and keep it as is.
What makes you think the motor needs a rebuild? You may just need a carb rebuild and be fine. Usually if the motor has been sitting over time with gas in it, the carb become gummed up from the gas going bad. Sometime you can get away with just a good cleaning.
Vic