Desk has some rust holes. Mower runs okay. Some said to use mondo putty
Or liquid steel. Any good fillers out there?

I assume you mean Bondo. For very small holes, liquid steel or JB weld is OK. Anything over 1/2" will fail with either of the above 2. I would use fiberglass resin and glass cloth. Last a lot longer then Bondo or JB weld.

I agree past a certain point the deck becomes structurally unsafe. A stone picked up by the blade can go right though many patches if not steel. I've removed the engine then found solid decks that the engine will fit (beware that not all engines fit all decks).

Thanks for the information on the craftsmen mower