So I bolted these new metal pieces to the engine, positioned it in place, and then tack welded the pieces into place so that I could remove the engine.

After welding the top, I gave it all some hits with a hammer to just to make sure they weren't bad welds that would fall apart easily.

I then didn't take any more photos until I was done with the welding.

This is the bottom.

Somehow this all shrunk the holes, so before I could get a bolt through them, I had to file them a little larger.

There's also a small crack that I forgot to weld that I didn't notice until I was putting stuff away.

I didn't think the welds looked too good at first, but in the last two days, I've been paying closer attention to welds I see in videos, and the ones I've seen do look better, but not nearly as much better as I thought.

Also I found the mower itself to be good practice. Like while working on it I figured out there's something to just welding for about 1.0 seconds, enough to get it hot enough to weld, but then letting it cool before continuing. I did that a lot to fill in gaps. For example, in the first photo you can see there's a gap behind the pieces of bar stock. I ended up cutting small pieces of steel plate to cover those gaps which fit in at an angle and then I kind of filled the whole thing with ... IDK what to call the wire that the machine spits out. It's not solder but kind of solder. Also I realized the rim of the hole was bent and I thought I should preserve that around my additions so I cut some strips and bent them along it and welded them to it. By the time I was done with it all I kind of felt like a welder.

I think it took about six hours. At least 3/4 of that time was cutting and cleaning what I was going to weld. The actual welding doesn't take too long.

Also about 90 minutes was spent finding new bolts. I realized I couldn't use one of the old ones because part of the deck was welded to it by the vibrations. I went to Lowes, then Mendards, then TSC where I found what I thought were just right, but then realized they lacked those serrated things on the bolt heads, so I went back to Lowes and bought their too-long bolts (which it turns out is fine, I don't even have to cut them) and then went back again for one more bolt because I decided I'd thread the 4th mounting hole in the engine which not only doesn't have a hole in the deck but the hole in the engine wasn't even threaded. I'm still not sure I can even use that one.

Since the bolts are too long anyway, and stick through the baseplate of the engine, I'm going to get some nuts to put on them, just because I'm not totally confident that the threads in the engine weren't damaged too. They seem OK, but the engine is aluminum, and since I can add nuts, I figure I might as well.

Anyway, it occurred to me that maybe we could have just bought a new deck for the mower. So I looked that up and it would have cost $90. Considering how long this all took, if my time were worth anything, that probably would have been the better option even if the welder were free. ...but, oh well, I have a new toy now I guess.