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 are the 716.649 or 716.654, which can handle over 500 HP. They 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. 
 
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 seperately.