I pretty much haven't welded since I did mine years ago, but it's not as bad as it might appear. I learned to weld
while doing my own engine bay, and was done within a few hours. Even the ugliest beads will do the trick, but it just means more prep work for paint. If you have access to a welder I'd highly recommend giving it a shot. I didn't think I'd be able to but my friend coaxed me into it and we got it done just fine.
You should get some 18-22g sheet metal to cut out filler shapes for the big holes, like you can see in my pic below. This was after grinding, I don't think I have any showing the actual weld beads I was working with. Some were good, some were rough, but they were strong so it was all good.