I put a new ladder portion on the rear sight of a Spanish Mauser the other night and it was a real pain in the butt to do by myself. If you are talking about depressing the leaf spring to get the sight and pin back in, you might just get someone to help you insert the pin while you are holding the spring down.
That is what I was going to do but finally I was able to hold the spring down with my right thumb and insert the pin with my left hand, I don't really know how I did it, but it just finally just slipped in as I was about to give up on it and put it away for the night.
I thought of the c-clamp idea too, but did not want to screw up the stock on the other side of the sight! I thought of trying a vise grip and again the thought of the wood made me forget that quick also. My best suggestion is just get someone to help you.
A suggestion that was given to me before I removed the old part, was to take something smaller and put in the hole to hold it there until I could put the roll pin in. I used a large paper-clip but I could not make it work.
I hope what I said makes sense to you, it does to me but then, I was there!