Piston replacement, 1994yz125

I'm going to replace the piston in my 94 yz125 and just wanted to see from more experienced people how hard it is. What exactly do i need. Im looking at a WISECO PRO-LITE PISTON KIT, but whats the difference between that and a WISECO PRO-LITE PISTON .020" OVERSIZE, etc etc. Is that if the cylinder has been bored. Also about exhaust bridging. How hard is it to drill the holes needed to make the piston work properly?


Thanks
Wes