Update:
Try to mount the sprocket in the weekend but it didn't fit in the chain! Was to wide, so yesterday I went to the garage and they take 1,5mm of sprocket metal and now it fits.
So mounted it last night and take it for a small test ride...
Results are amazing: it is so much better, I now can give some use to 2nd gear, 1st and 2nd gear are smoother when changing gears, now it goes so much better, top speed much better also, feels like a bigger bike, and don't feel so much the lack of more gears...
when uphill in 1st and 2nd didn't notice any difference (could notice big difference if I take a passenger I supposed, but "I ride alone") the only thing I felt more was when in 3 gear and pull the throttle to max, at low revs it goes well, but in the mid rpm range it slows a bit, then entering high rpm it goes better again... I think it happens before, but now it is more noticeble...
Also I'm using for the 1st time a 95 "regular" petrol (the pump didn't have the premium) and with this petrol the engine is noisy, pulls worst, and make more smoke. With 98 octane premium is so much better. Can wait to waist this tank to putt premium again
(in 3 liters I put every tank, the cost of premium petrol over regular is peanuts, and it makes a big difference too)
Conclusion: If it was possible to add 2T the bikes behaviour would suffer a lot.
As it was, the gearing was to much short, maybe because of some extra hp to 75cc rebuild.
As it is now is perfect, the gear are a right match to engine real "performance". So couldn't be happier, because after all, it will give me some more mpg too!
Amazing how one extra T on the sprocket transforms it into a completely diferent bike!