I have some large 3/4" sockets - I don't remember the size of that nut.

Clamp the clutch down with c-clamps using some wood and loosen the nut. When reassembling, you will be able to compress the spring by hand and clamp it with the c-clamp before you get the nut back on.

i don't know if this will help but on my friends 250cc for the nut that held the clutch on we used a 1 9/16 socket it was tight and had to tap it on the nut.the 1 5/8 socket was just a little loose for me i thought and didn't want to round it off
but i think if the nut is the same on yours either one will work.
the sockets where 12 point we used and had some wear in them.
pud

Well once i put a vise together,it wasnt to bad.17mm was the size i think.i put in a quarter inch shim and stood on it to first start the thread.well i learn about the clutch tonight!slipping the whole thing and the belt back on was interesting,i now know about those rollers,yep a few fell down.my investment in a half inch drive electric impact was worth it.

sorry about that i thought you where shimming the spring on a 250cc
well i guess the main thing is you got it the big nut that holds my clutch and spring on my 150cc is 1 1/2
pud

I was curious if you would get similar results if you changed to a 1500 RPM torque spring? Wouldn't the larger spring limit the amount of pulley travel like the shim does? The reason I ask is because I have already changed the torque spring and I was wondering if there would be any benefit if I added a shim as well or if that would limit the pulley travel too much. My reasoning is that a 1500 RPM spring when fully compressed might be 5/16 larger than the stock spring fully compressed therefore you might get similar results to adding a shim.