A liquid form of superglue applied very carefully using a needle with a tinyl drop on each corner of the frame. use a pair of tweezers to place the mirror into the frame and wear surgical gloves to lightly press the edges to seat the mirror.

Anything thicker than liquid glue may shift your mirror out of alignment and upset the focussing. Not by much but not right.

I did the same to a Minolta SRT101b years ago and it still is in place today.