Best bet is get a Dax Gen 4 engine and paint it black... Now you got a high quality smooth running engine that's just begging for more upgrades and can withstand the extra power, then mask off the ports and spark plug hole then paint it black with some engine block paint.
The black engine kits are all just painted versions of the same engines and the finish quality is usually poor at best... with free overspray in the cylinders and ports where it doesn't belong so you're really best off ordering a silver engine and painting it yourself.
After buying a few kits, the only engines I'll buy are from Dax or MotorBicycleRacing, The Dax engines are available from his web site linked on the side of these pages, or you can PM or email MotorBicycleRacing and order one of his engines, both these sellers have the highest quality PK 80 type engines you can get, and probably for less than most ebay kits too.