First off you will need to buy a new flint lock to replace the percussion one because it is nowhere near cost effective to convert the percussion lock to flint. If you can do that then there is the barrel to contend with. If the powder drum simply screws into the side of the barrel then you can unscrew it, replace it with a proper thread-size vent liner insert and hope the flash hole aligns with the pan on the new lock. However, if the barrel has a patent breech or snail-style breech plug then you are faced with removing the snail or whole assembly from the barrel, fitting a new breech plug and tang, re-inletting the rear of the barrel and tang into the stock, re-positioning the barrel underlugs that hold the barrel in the stock, drilling a flash hole in the proper location and if it is a full stock then shortening the fore-stock to match the reduced barrel length.
Hope you are up for a challenge.
If you are new to BP and PRB shooting then I strongly suggest you spend a year learning the intricacies of getting performance out of a percussion then buy a good quality flinter and carry on. Also, while learning about your percussion, go to the National Muzzle Loading Rifle Association website and buy the new $30 book by Eric Bye on Flintlocks - it will save you years of frustration on the flintlock learning curve and a lot of what he has to say also applies to getting the most out of percussion shooting. Good luck and have fun.