Can i weld up a quarter sized hole on my valve cover?

Sure can, Either machine up a bung to go inside it and weld/grind it flush or just patch over it..

Best is to make up a filler aluminum piece that's close to the size of the hole, and then start welding.

Yes you can but it all depends on your skill level.

I personaly would not. Im not big into flashy things like chrome rims or glow in the dark paint jobs but I like my engine bay to be nice and spiffy but no flash and if you are like the rest of us and strapped for cash these days you can. Start by grinding the whole to as close to a square as you can get it, spot a piece into the whole, run a small bead from both sides grind paint and wa la presto chango!!! I am the run on sentance king!!!!