Park it on a reasonable hill without the brake on
then start pulling the mower backwards,
If the tranny is good you should feel strong resistance to rolling.
If it starts rolling easily & gets a faster quickly, then the tranny is shot.
My test is my tilting trailer.
If the mower rolls strait off the tranny is shot.
If I have to drag it off then the tranny is good.
Very rare for one to pack it in at those hours.
And the tranny is a K 46, marked NLA from most suppliers.