A motorcycle transmission, is about the easiest cheapest way.
A motor, broken or otherwise not good motor with a good unitized transmission is also an option. It however will need work to do the conversion for the motor delete.
I am not any expert on the subject but as to my knowledge, typically speaking the case needs to have the cylinder(s) removed and blanked off with a plate and sealed. Depending on the motor, the trans will need some way of coupling power intake to the trans, externally.
Typically the trans uses motor oil to lube, so a pump and other stuff is needed.
That's what I know off hand.