This 1988 BMW E24 M6 is for sale on eBay (ESD may earn commisions when a product is purchased from this link) with a starting bid of $22,000 or buy-it-now of $25,750. The previous owner didn’t enjoy the power output of the 3.4 L S38B35 inline-six which made only 256 horsepower and 243 lb-ft of torque in North America. So they swapped it for a 3.5 L S38B36 inline-six from a 1991 E34 M5 which produced 315 horsepower and 266 lb·ft of torque. The owner left the rest of the drivetrain factory.
Source: eBay (ESD may earn commisions when a product is purchased from this link)