I bought a "low scope safety " for my yugo.after installation the safety can not be put on.Isnt there some mod that is required,like machining the top of the cocking piece?I thought this was a drop in installation.maybe I should get some of the length of the firing pin fall removed at the same time?

I assume this is one of the left side safeties off E-Bay.
I would not alter the cocking piece except as a last resort. You will be exposing soft metal if it is casehardened.
Instead look at the rear of the safety. You should see the camming surface in the lower left. Gently alter the left edge so that it can slide in front of the cocking piece when lifting the safety to the "on" position.

The relationship between the cocking piece and the safety on the Yugo mauser and the 98 is different. The firing pin fall is shorter on the Yugo to facilitate a faster lock time. The only way to make that safety fit is to mill the top of the cocking piece, usually about 1/8".

Is it possible to mill the safety instead of the cocking piece?wouldnt milling the cocking piece make be a one way street,as in the original safety wouldnt work anymore,if i wanted to put it back on at some point?