Seized engine.

These engines are simple, like briggs and stratton simple, except they are water cooled.
Check the basics, is the coolent full, extra fluid in the oil pan?
These engines run HOT at anything more than 35 mph.
Pull the plugs and see if you have anti-freeze in there, head gaskets are not the strong point of these motor.
What was the oil pressure gauge reading when it seized, mine runs 60 cold and 30 hot.