I think that one has to separate out the obvious value here first to decide on whether to pursue this or not.
The LT-26 hitch is difficult to find as a used item because you just don't see people parting out loader tractors very often. If a J-26 hitch brings $400.00 on average, then I think that a LT-26 should be worth $500.00 in the marketplace of today.
Add in another $125.00 for the F-27 adapter and $300.00 for the wheel weights and $200.00 for the equally rare rear PTO kit for the loader tractors and you already have $1125.00
So, at the most.....we are talking about a 600 series with two complete engines for the asking price of $975.00 and we know that he'll take 2 grand anyway for it. For sure, a 646 is worth $875.00 all day long, especially when it's in that state of condition.
What's in question are the engines. Is one of them the original CCKA? Are they both CCKA's or is one of them a B series?
To me, anyone looking at this package should want to keep it for a long time. He's already paying two grand for a non-running machine and the engines are a pig-in-a-poke. I would do a thorough examination of the "running engine" prior to installing it... to the point of nailing it to a bench and test running it. I'd want to know how much compression it had after I ran it for 15 minutes at various throttle points and I'd also want to know if it smoked visibly and how badly.
If that engine had decent compression (90 or better) and did not huff blue clouds, then I'd gamble on it and install it. I would then take the original engine apart, at my leisure, perform a thorough rebuild and test run it before mothballing it for the interim. As long as the donor engine continued to work for me, I'd wring every ounce of life out of it. Why not? This is a work tractor, not a trailer queen resto project. Whenever the donor engine proved to be worn out, then I'd swap the original engine back into the frame.