upgrading crankshaft oil seals

I suppose cutting it down would resolve this. But some sources say its to adjust the tension of the internal crank shaft as i find highly unlikely due to it just being a bolt rubbing against the internal components.
The clutch shaft spring collar has several slots cut in it. You can take the cable holder out, stick a flat blade screw driver in the hole, get the driver in one of those slots, and adjust the tension that way.
Some where on this site there’s detailed instructions on how it’s done.
The short answer is once you have a pin punch or narrow screw driver holding the slot in place, push the bike backwards 6 inches or so will tighten the clutch preload spring. Don't over do it.