FWIW
Now I am back & have time to check, this is a fuel injected engine & is obsolete
The recommended replacement is
ECH749-3EXM and even then this is not a direct replacement & requires a certain amount of "fitting".
So this "gift" might not be a generous as first looked
It can be replaced with a cheaper carburettor engine but that will require reworking of the switches .
Any of the 700 series Kohlers should fit CH742 would be the closest
That will be the cheapest route if the engine is not repairable.
As you will not be using it 10 hours a day running the lower power carburettor engine should not be a problem but again it will not be cheap
to replace the engine still looking at $4000 to $ 6000
I don't know if any of the engines I can access will fit as this engines uses both a standard PTO & a flywheel PTO so exact dimensions are important
Sorry I should have checked it properly before I replied
Download the Kohler manual
The EFI mower engines like EFI car engines will send you fault codes .
As for repairing the engine, that will depend upon what is wrong
We can probably walk your through repairing the engine but fitting a different one is another matter
I have done all sorts of strange things to keep customers mowers running but that was when it was in my workshop
Doing it on the web is a whole different matter
The owners manual listed Kawasaki engines so there must be a Kawasaki auxilary PTO kit that will fit but I have no idea where to get one from in Qld as I am in Syd .
But we are here to help , particularly for a good cause ( I was in a troup myself till work pressures became too much ) .