Honda Twin GXV 610 meltdown

I volunteer for a local non-profit, and one of the other volunteers was mowing with the zero turn Fastrak (spelling?) unit, when it got grass clogged on the cooling intake, and overheated badly. Not only did it stall, but the dipstick tube melted as did the air cleaner case. It will run, but it clatters badly. The not-very-local dealer has a huge backlog, so we're considering a DIY repair or an DIY engine transplant (to a GXV 630).

What typically needs replacement after a bad overheat? I'm guessing the valves, cam bearings, maybe main bearings, crank bearing, rings, and cylinder sleeves. Is it a hopeless rabbit hole or just a lot of labor and parts? I've replaced crankshafts on many mowers, so I'm not a complete newb. A new 630 is about $1200.

Sorry to hear that.
Minimum will be the entire head + all the plastic parts.
Usually they are cheaper as a direct replacement but a bare head will do .
Valves should be fine but any plastic guides will be distorted.
Pull the blower housing off and adjust the valve lash, it will most likely be massive causing the rattle.
Start the engine,if it is not noticably quieter then the con rod is gone and possibly the piston as well.
If oil was circulating the crank should be fine.
The damage will be to the head piston bore and little end if it was just overheating ( could also have been a fire )
However if the oil ended up being way too low then you are looking at a new engine as there are no bearings, the crank runs directly on the cases .
They can be bushed but unless you have someone willing to do free machining the cost is way too expensive comparred to replacing the engine.

Let us know what you find

Two things to keep in mind when replacing that Honda. Depending on year, Honda made changes to the V-twin engines a few years ago that resulting in the angle of the exhaust being different which translates into the old exhaust may not fit the new engine, Also they went from a open to run ignition system to a close to run, which could require changing the key switch also.