I weighed exactly the same when I had a brand new 2000. The bike came very soft front and rear. You could add a spacer in the left fork as they only had one spring. I would just ride it as is. In trials it is better to be soft than hard. Sag really has no place in trials.
You do have the gas valve in the correct position? It is going to leak a little doing tricks. If you block the vent tube, it is not going to run. Take it to someone that can fix it. You are making it worse.
All that increases is the rpm for the same speed. Throw a 9t cs. until you can find a nice tame 250 Beta. You will be surprised how much mellower they ride.
I have never done an Ossa, but lots of gassers. Nice video, but you never spun the gears by hand to see if they move. And the cassette of gears are probably not fully set into the center case is why the clutch fingers are hitting.