@Dave I. Even though we haven't met, everything I've heard about you has led me to believe that you're the kind of person (skills, temperament) who could probably tackle this. Lots of positive comments from WW'ers. I do think it'd be difficult (for anyone) to take it on as a side-gig. Dan's operation was a full-time business all by itself. If you can't, or don't want to, restructure your current life/obligations, then I like
@PowellBride's suggestion. Maybe you could portion off some slice of Dan's former business model that makes sense according to your availability (off-season, previously scheduled, etc)?
It'd still be really great if someone took over the on-demand repair portion of his business. In my opinion, this was by far the most attractive and valuable offering. And, as has been stated earlier, could also be the most lucrative. This service has saved untold numbers of vacations over the years. When you've spent several thousand dollars on food, fuel, houseboat shares & dues, jetski rentals, etc, for seven days on the lake with extended family and friends, only to find the generator won't start, you're willing to go to great lengths ($$$) to save the trip.
This exact scenario played out for our family a couple of years ago. Dan answered the phone after hours and was on our boat early the next morning. He found a cracked fuel supply line that was allowing air bubbles into the fuel. It took him about 30 minutes to replace it and get us all patched up. His price was reasonable, but we would have gladly paid considerably more to save the huge investment already made in one of our bigger trips.
Only you can decide if that's something you want to get into. Either way, thanks for all of the help and support you've given us Lake Powell aficionados over the years. Good luck!