Well I got the replacement fixture last Friday and it worked perfectly. I modified it to a remote ballast configuration and have been running it this week for my tomatoes. It seems to be doing great.
I read that this particular model draws a lot of current because it lacks the typical capacitor of most ballast configurations, so I ordered a capacitor that ought to do the job and bring it up to a nice high efficiency. That arrived today and, thanks to the difficulty of determining size in an online picture, is much larger than I’d anticipated (about the size of a juice can). I’m not entirely sure how well that will work with my current rig, but I’ll figure something out.
Once I do I’ll whip up a complete “How To DIY” based on this fixture, including where I got all my stuff. HPS on a budget.