Most engines will fit in any tractor -  flatheads and OHV motors have different patterns tho.
From the factory most tractors come with a variety of different pre drilled bolt holes, due to different models using different motor combos. 
Only things that might be different are engine widths ( opposed motors are wider then Vtwins) and wiring.
Id suggest using the original motor's engine pulley so it wont affect the drivetrain/mower deck speeds.