Manual six speed transmission that bolts right up to your OM601-OM606 as well as M111 and M10X engines. In used, good working condition.
 
These had to be imported from Europe, since they were never sold in the US. They are from CDI's so the gearing is for diesel engines and they are very stout. One of our customers uses one in his W210 with 170cc of fuel and 3 bar of boost! 
 
There are cheaper manual transmissions out there, but you'll need an adapter plate kit, which are usually over $1,000. These bolt right up. No modifications or adapters needed.
 
Clutch kit, pedals and shifter are available separately.