I just want to make some updates:
Today, based on information from you friends, I found and changed the spark plug for b5hs, and put some more mileage on the bike. As this spark plug has a couple of thread wires more then the stock one, I ended up putting an extra washer to avoid the risk of hiting the piston. I can say that now I´m starting to have fun.
Apparently the engine is showing signs of breaking in - not so much in max speed but in the way it is facing uphills. Even the sound of the engine is changing. I also noticed that with the changing of the spark plug, the engine became more predictable, both in starting and in riding. It is also working a little cooler I think.
With the exception of the leaked oil I found when arrived (I must have put too much oil in the crankcase before mounting it on the bike, and now I believe that with rotation and heat engine is starting to throw it out), I'm much more satisfied with the result.
Thank you so much to all the advices!