I got the same measurements as nath ,the 11 hp was too small and the 12.5 hp was 10 mm too long ,I was thinking the same thing that the head may have been swapped . I would have just used 10 mm spacers on the front 2 bolts or 10 mm alloy flat bar.
The head swap may have been to raise the comp ratio as the compression may have gone down from age ,unless it was rebuilt and you can trust the seller.
Mowers can be found with good engines from $50 to around a $100 but as Norm said if you have to buy parts or spend time fixing an engine it's more cost effective to buy a new motor.
