As a commercial venture a no no,though I assume you have access to a good university workshop so machining shouldnt be a problem and if your design uses straight cut gears again a simple machining operation, though the correct heat treatment is vital and should be sent to a specialist in heat treatment. As for castings they are not that expensive in low temp alloys though casting steel is a different matter.
I think most people are afraid of this sort of project due to the inability to workout loading both shock and running, gear design spec meshing depth etc etc.
Do it!(I am ex drivetrain engineer, R&D by the way)