Riding lawn mowers

Dad has early 70's Gravley, rider great machine, last year we replaced the Koler engine with a 22 HP v-twin Honda, its a beast of a mower.
I bought him a new John Deere for is birthday and he said, I guess you are not working on my Gravely no more.
That Gravely is tough, he wore out the original engine, bought a new replacement Koler, wore it out, then I put the Honda engine on it.
I bet it the old rider had an hr meter on it, it would be over 20,000 hrs.