Mechatronik is a company located in Pleidelsheim, Germany that builds classic Mercedes coupes or sedans with modern powertrains. Take for example this 1969 Mercedes-Benz W113 280 SL which originally came with a 2,778 cc (2.8 L) M130 inline-six. Mechatronik replaced that engine with a 4.3 L M113 E43 V8 from a W202 C43 AMG. The ad states the engine produces 290 hp but other sources list the AMG V8 produces 302 hp and 302 lb-ft of torque. Other amenities installed was a modern automatic transmission, anti-lock brakes and traction control. Of course these upgrades don’t come cheap. The coupe is listed for €299,000 or about $322,232.
This video is of another M-SL but it gives you an idea of the wonderful sound that this car would make.