I've just done this conversion myself although as I dont have an engine in the bike at the moment I havent tried it yet.
The spline on the gear lever is the same. I made that part up out of an original arm as that was what I had. I made it the same length so no change to the leverage there.
Having a look I think you could use a gear lever turned around. You would need to shorten it and weld a tab for the rod to attatch to on the end.
I made the lever for the rear drum 1/2" longer. The one thing I did do was to make the angle of both levers so they sat at about 11 o'clock, they are at about 12 o'clock with the brake hard on. I also fitted a longer torque arm so the pivot point on the rear drum is slightly higher than the axle. I am hoping all this will give me slightly better geometry. Since I took these pics I have turned the torque arm into a "dogbone" and tidied it all up so it looks a bit more original. I made it all out of stainless.
The pedal definitely feels better, it goes a lot harder when the brakes are on, not so spongy. As soon as I get it going again I will know if it was worth doing.