How to fill in gaps between window glass and frames?

I have an 100 year old Queenslander with a few of the original casement windows left. I’ve been working on sealing up all the gaps letting in cold air and realised some of the window glass pieces are too small/broken/shifted and letting in cold air (and probably bugs I suppose). I just used blu-tack to cover the worst gaps but I was hoping for something a little less ugly. I’ve got silicone, would that work? I’m headed to the big green shed tomorrow so is there something better? My DIY skills are rudimentary and eventually the windows will be replaced so it doesn’t have to be fancy.

Thanks