That's a bummer.
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However rebuilding that engine during the off season can be some quality father son time.
In the mean time a "daddy bank" loan for a replacement engine might be in order.
Lots of cheap engines this time of year on the surplus engine sites
Surplus centre
Small Engine warehouse
Small engine Supplies
Hp ratings +/- 15% will be insignificant if he is not mowing 8 hours a day.
QALthough going with the same engine will save hassels with mufflers.
IF he has the 1.125' CRANK
https://www.smallenginesuppliers.com/shop/html/pages/products/briggs_vertical_shaft_engines4395.html
SAME PRICE FOR THE 1" CRANK
https://www.smallenginesuppliers.com/shop/html/pages/products/briggs_vertical_shaft_engines4394.html
If the mower is in good nick he will be set for decades
or he can sell it "with a brand new engine" ata price that reflects this and hopefully get out unburned.