I am not in a position to know or comment on the second hand/non running value of a Toro 7453.
It would appear (Google) that the new replacement cost of a 7453 ,is north of $10,000 US. If correct , your cost of replacing the engine at $1500 + , may be a cost effective solution.
I point out, even with the very low $650 ($?) valuation, the mowere may be worth significantly more in going condition AND will cost the owner a lot more to replace with a new unit of similar capability.
I am not
"preaching" to you, just advising the owner to explore their options, which did include cost & effect.